Interconnects for solar cell devices

ABSTRACT

A solar cell assembly comprising a plurality of elongated solar cells, each respective solar cell comprising (i) a core configured as a first electrode, (ii) a semiconductor junction circumferentially disposed on the core, (iii) a transparent conductive oxide (TCO) layer disposed on the semiconductor junction, and (iv) an elongated counter-electrode disposed lengthwise on a first side of the respective solar cell and extending outward from the TCO layer. On a second side of each cell, approximately opposite the counter-electrode, is a notch or other disruption extending through the semiconductor junction and the transparent oxide layer, thereby exposing the core of the solar cell. The solar cell assembly may further comprise conductive internal reflectors configured between a first and second elongated solar cell in the plurality of solar cells such that a portion of the solar light reflected from the respective internal reflector is reflected onto the solar cells.

1. FIELD OF THE INVENTION

This invention relates to solar cell assemblies for converting solarenergy into electrical energy and more particularly to improved solarcell assemblies.

2. BACKGROUND OF THE INVENTION

Interest in photovoltaic cells has grown rapidly in the past fewdecades. Photovoltaic cells comprise semiconductor junctions such as p-njunctions. It is well known that light with photon energy greater thanthe band gap of an absorbing semiconductor layer in a semiconductorjunction is absorbed by the layer. Such absorption causes opticalexcitation and the release of free electrons and free holes in thesemiconductor. Because of the potential difference that exists at asemiconductor junction (e.g., a p-n junction), these released holes andelectrons move across the junction in opposite directions and therebygive rise to flow of an electric current that is capable of deliveringpower to an external circuit. The flow of carriers into the externalcircuit constitutes an electrical current density, J amp cm⁻², which,under short-circuit conditions, is known as the short-circuit currentdensity, J_(sc). At the same time, the separation of the charges (holesand electrons) sets up a potential difference between the two ends ofthe material, φ, which under open circuit conditions is known as theopen-circuit voltage, φ_(OC). It is desirable to maximize both J_(sc)and φ_(OC). For interaction with the solar spectrum, J_(sc) and φ_(OC)are optimized when the junction semiconductor absorber has a band gap ofabout 1.4 electron volts (eV).

It is presently common practice to provide an array of solar cells togenerate electrical energy from solar radiation. Many solar cells aremade of silicon. However, cells made of other materials, e.g., cadmiumsulfide and gallium arsenide, have also been developed and tested.Crystalline silicon has traditionally been a favored material since ithas a band gap of approximately 1.1 eV and thus favorably responds tothe electromagnetic energy of the solar spectrum. However, because ofthe expense in making crystalline silicon-based cells, thin film solarcells made of materials other than silicon have been explored and used.

Presently solar cells are fabricated as separate physical entities withlight gathering surface areas on the order of 4-6 cm² or larger. Forthis reason, it is standard practice for power generating applicationsto mount the cells in a flat array on a supporting substrate or panel sothat their light gathering surfaces provide an approximation of a singlelarge light gathering surface. Also, since each cell itself generatesonly a small amount of power, the required voltage and/or current isrealized by interconnecting the cells of the array in a series and/orparallel matrix.

A conventional prior art solar cell structure is shown in FIG. 1.Because of the large range in the thickness of the different layers,they are depicted schematically. Moreover, FIG. 1 is highly schematic sothat it represents the features of both “thick-film” solar cells and“thin-film” solar cells. In general, solar cells that use an indirectband gap material to absorb light are typically configured as“thick-film” solar cells because a thick film of the absorber layer isrequired to absorb a sufficient amount of light. Solar cells that use adirect band gap material to absorb light are typically configured as“thin-film” solar cells because only a thin layer of the direct band-gapmaterial is needed to absorb a sufficient amount of light.

The arrows at the top of FIG. 1 show the direction of the solarillumination on the cell. Layer (element) 102 is the substrate. Glass ormetal is a common substrate. In thin-film solar cells, substrate 102 canbe-a polymer-based backing, metal, or glass. In some instances, there isan encapsulation layer (not shown) coating substrate 102. Layer 104 isthe back electrical contact for the solar cell. It makes ohmic contactwith the absorber layer of semiconductor junction 106.

Layer 106 is the semiconductor absorber layer. In many but not all casesit is a p-type semiconductor. Absorber layer 106 is thick enough toabsorb light. Layer 108 is the semiconductor junction partner-thatcompletes the formation of a p-n junction, which is a common type ofjunction found in solar cells. In a solar cell based on a p-n junction,when absorber 106 is a p-type doped material, junction partner 108 is ann-type doped material. Conversely, when layer 106 is an n-type dopedmaterial, layer 108 is p-type doped material. Generally, junctionpartner 108 is much thinner than absorber 106. For example, in someinstances junction partner 108 has a thickness of about 0.05 microns.Junction partner 108 is highly transparent to solar radiation. Junctionpartner 108 is also known as the window layer, since it lets the lightpass down to absorber layer 106.

In a typical thick-film solar cell, layers 106 and 108 can be made fromthe same semiconductor material but have different carrier types(dopants) and/or carrier concentrations in order to give the two layerstheir distinct p-type and n-type properties. In thin-film solar cells inwhich copper-indium-gallium-diselenide (CIGS) is the absorber layer 106,the use of CdS to form layer 108 has resulted in high efficiency cells.Other materials that can be used for layer 108 include, but are notlimited to, SnO₂, ZnO, ZrO₂ and doped ZnO.

Layer 110 is the top transparent electrode, which completes thefunctioning cell. Layer 110 is used to draw current away from thejunction since junction partner 108 is generally too resistive to servethis function. As such, layer 110 should be highly conductive andtransparent to light. Layer 110 can in fact be a comb-like structure ofmetal printed onto layer 108 rather than forming a discrete layer. Layer110 is typically a transparent conductive oxide (TCO) such as doped zincoxide (e.g., aluminum doped zinc oxide), indium-tin-oxide (ITO), tinoxide (SnO₂), or indium-zinc oxide. However, even when a TCO layer ispresent, a bus bar network 114 is typically needed to draw off currentsince the TCO has too much resistance to efficiently perform thisfunction in larger solar cells. Network 114 shortens the distancecharger carriers must move in the TCO layer in order to reach the metalcontact, thereby reducing resistive losses. The metal bus bars, alsotermed grid lines, can be made of any reasonably conductive metal suchas, for example, silver, steel or aluminum. In the design of network114, there is design a tradeoff between thicker grid lines that are moreelectrically conductive but block more light, and thin grid lines thatare less electrically conductive but block less light. The metal barsare preferably configured in a comb-like arrangement to permit lightrays through TCO layer 110. Bus bar network layer 114 and TCO layer 110,combined, act as a single metallurgical unit, functionally interfacingwith a first ohmic contact to form a current collection circuit. In U.S.Pat. No. 6,548,751 to Sverdrup et al., hereby incorporated by referencein its entirety, a combined silver (Ag) bus bar network andindium-tin-oxide layer function as a single, transparent ITO/Ag layer.

Layer 112 is an antireflection (AR) coating that can allow a significantamount of extra light into the cell. Depending on the intended use ofthe cell, it might be deposited directly on the top conductor (asillustrated), or on a separate cover glass, or both. Ideally, the ARcoating reduces the reflection of the cell to very near zero over thespectral region in which photoelectric absorption occurs, and at thesame time increases the reflection in the other spectral regions toreduce heating. U.S. Pat. No. 6,107,564 to Aguilera et al., herebyincorporated by reference in its entirety, describes representativeantireflective coatings that are known in the art.

Solar cells typically produce only a small voltage. For example, siliconbased solar cells produce a voltage of about 0.6 volts (V). Thus, solarcells are interconnected in series or parallel in order to get areasonable voltage. When connected in series, voltages of individualcells add together while current remains the same. Thus, solar cellsarranged in series reduce the amount of current flow through such cells,compared to analogous solar cells arrange in parallel, thereby improvingefficiency. As illustrated in FIG. 1, the arrangement of solar cells inseries is accomplished using interconnects 116. In general, aninterconnect 116 places the first electrode of one solar cell inelectrical communication with the counter-electrode of an adjoiningsolar cell.

As noted above and as illustrated in FIG. 1, conventional solar cellsare typically in the form of a plate structure. Although such cells arehighly efficient when they are smaller, larger planar solar cells havereduced efficiency because it is harder to make the semiconductor filmsthat form the junction in such solar cells uniform. Furthermore, theoccurrence of pinholes and similar flaws increase in larger planar solarcells. These features can cause shunts across the junction.

A number of problems are associated with solar cell designs present inthe known art. A number of prior art solar cell designs and some of thedisadvantages of each design will now be discussed.

As illustrated in FIG. 2, U.S. Pat. No. 6,762,359 B2 to Asia et al.discloses a solar cell 210 including a p-type layer 12 and an n-typelayer 14. A first electrode 32 is provided on one side of the solarcell. Electrode 32 is in electrical contact with n-type layer 14 ofsolar cell 210. Second electrode 60 is on the opposing side of the solarcell. Electrode 60 is in electrical contact with the p-type layer of thesolar cell. Light-transmitting layers 200 and 202 form one side ofdevice 210 while layer 62 forms the other side. Electrodes 32 and 60 areseparated by insulators 40 and 50. In some instances, the solar cell hasa tubular shape rather than the spherical shape illustrated in FIG. 2.While device 210 is functional, it is unsatisfactory. Electrode 60 hasto pierce absorber 12 in order to make an electrical contact. Thisresults in a net loss in absorber area, making the solar cell lessefficient. Furthermore, such a junction is difficult to make relative toother solar cell designs.

As illustrated in FIG. 3A, U.S. Pat. No. 3,976,508 to Mlavsky disclosesa tubular solar cell comprising a cylindrical silicon tube 2 of n-typeconductivity that has been subjected to diffusion of boron into itsouter surface to form an outer p-conductivity type region 4 and thus ap-n junction 6. The inner surface of the cylindrical tube is providedwith a first electrode in the form of an adherent metal conductive film8 that forms an ohmic contact with the tube. Film 8 covers the entireinner surface of the tube and consists of a selected metal or metalalloy having relatively high conductivity, e.g., gold, nickel, aluminum,copper or the like, as disclosed in U.S. Pat. Nos. 2,984,775, 3046324and 3005862. The outer surface is provided with a second electrode inthe form of a grid consisting of a plurality of circumferentiallyextending conductors 10 that are connected together by one or morelongitudinally-extending conductors 12. The opposite ends of the outersurface of the hollow tube are provided with twocircumferentially-extending terminal conductors 14 and 16 that interceptthe longitudinally-extending conductors 12. The spacing of thecircumferentially-extending conductors 10 and thelongitudinally-extending conductors 12 is such as to leave areas 18 ofthe outer surface of the tube exposed to solar radiation. Conductors 12,14 and 16 are made wider than the circumferentially-extending conductors10 since they carry a greater current than any of the latter. Theseconductors are made of an adherent metal film like the inner electrode 8and form ohmic contacts with the outer surface of the tube. While thesolar cell disclosed in FIG. 3 is functional, it is also unsatisfactory.Conductors 12, 14, and 16 are not transparent to light and therefore theamount of light that the solar cell receives is reduced.

U.S. Pat. No. 3,990,914 to Weinstein and Lee discloses another form oftubular solar cell. Like Mlavsky, the Weinstein and Lee solar cell has ahollow core. However, unlike Mlavsky, Weinstein and Lee dispose thesolar cell on a glass tubular support member. The Weinstein and Leesolar cell has the drawback of being bulky and expensive to build.

Referring to FIGS. 3B and 3C, Japanese Patent Application KokaiPublication Number S59-125670, Toppan Printing Company, published Jul.20, 1984 (hereinafter “S59-125670”) discloses a rod-shaped solar cell.The rod shaped solar cell is depicted in cross-section in Figure. Aconducting metal is used as the core 1 of the cell. A light-activatedamorphous silicon semiconductor layer 3 is provided on core 1. Anelectrically conductive transparent conductive layer 4 is built up ontop of semiconductor layer 3. The transparent conductive layer 4 can bemade of materials such as indium oxide, tin oxide or indium tin oxide(ITO) and the like. As illustrated in FIG. 3B, a layer 5, made of a goodelectrical conductor, is provided on the lower portion of the solarcell. The publication states that this good conductive layer 5 is notparticularly necessary but helps to lower the contact resistance betweenthe rod and a conductive substrate 7 that serves as a counter-electrode.As such, conductive layer 5 serves as a current collector thatsupplements the conductivity of counter-electrode 7 illustrated in FIG.3C.

As illustrated in FIG. 3C, rod-shaped solar cells 6 are multiplyarranged in a row parallel with each other, and counter-electrode layer7 is provided on the surface of the rods that is not irradiated by lightso as to electrically make contact with each transparent conductivelayer 4. The rod-shaped solar cells 6 are arranged in parallel and bothends of the solar cells are hardened with resin or a similar material inorder to fix the rods in place.

S59-125670 addresses many of the drawbacks associated with planar solarcells. However, S59-125670 has a number of significant drawbacks thatlimit the efficiency of the disclosed devices. First, the manner inwhich current is drawn off the exterior surface is inefficient becauselayer 5 does not wrap all the way around the rod (e.g., see FIG. 3B).Second, substrate 7 is a metal plate that does not permit the passage oflight. Thus, a full side of each rod is not exposed to light and canthus serve as a leakage path. Such a leakage path reduces the efficiencyof the solar cell. For example, any such dark junction areas will resultin a leakage that will detract from the photocurrent of the cell.Another disadvantage with the design disclosed in FIGS. 3B and 3C isthat the rods are arranged in parallel rather than in series. Thus, thecurrent levels in such devices will be large, relative to acorresponding serially arranged model, and therefore subject toresistive losses.

Referring to FIG. 3D, German Unexamined Patent Application DE 43 39 547A1 to Twin Solar-Technik Entwicklungs-GmbH, published May 24, 1995,(hereinafter “Twin Solar”) also discloses a plurality of rod-shapedsolar cells 2 arranged in a parallel manner inside a transparent sheet28, which forms the body of the solar cell. Thus, Twin Solar does nothave some of the drawbacks found in S59-125670. Transparent sheet 28allows light in from both faces 47A and 47B. Transparent sheet 28 isinstalled at a distance from a wall 27 in such a manner as to provide anair gap 26 through which liquid coolant can flow. Thus, Twin Solardevices have the drawback that they are not truly bifacial. In otherwords, only face 47A of the Twin Solar device is capable of receivingdirect light. As defined here, “direct light” is light that has notpassed through any media other than air. For example, light that haspassed through a transparent substrate, into a solar cell assembly, andexited the assembly is no longer direct light once it exits the solarcell assembly. Light that has merely reflected off of a surface,however, is direct light provided that it has not passed through a solarcell assembly. Under this definition of direct light, face 47B is notconfigured to receive direct light. This is because all light receivedby face 47B must first traverse the body of the solar cell apparatusafter entering the solar cell apparatus through face 47A. Such lightmust then traverse cooling chamber 26, reflect off back wall 42, andfinally re-enter the solar cell through face 47B. The solar cellassembly is therefore inefficient because direct light cannot enter bothsides of the assembly.

Discussion or citation of a reference herein will not be construed as anadmission that such reference is prior art to the present invention.

3. SUMMARY OF THE INVENTION

One aspect of the present invention provides a solar cell assemblycomprising a plurality of elongated solar cells. Each elongated solarcell in the plurality of elongated solar cells comprises (i) aconductive core configured as a first electrode, (ii) a semiconductorjunction circumferentially disposed on the conductive core, (iii) atransparent conductive oxide layer disposed on the semiconductorjunction, and (iv) an elongated counter-electrode disposed lengthwise ona first side of the solar cell and extending outward from thetransparent conductive oxide layer. On a second side of each solar cell,approximately opposite the counter-electrode, is a notch or otherdisruption extending through the semiconductor junction and thetransparent oxide layer, such that the conductive core of the solar cellis exposed. The elongated solar cells in said plurality of elongatedsolar cells are geometrically arranged in a planar array, with eachsolar cell parallel or near parallel to adjacent solar cells. Theelongated solar cells are electrically connected in series, wherein thecounter-electrode of each solar cell extends into the notch of anadjacent solar cell and contacts or is otherwise in electricalcommunication with the conductive core of the adjacent solar cell. Tocomplete the circuit, a lead or wire may connect the counter electrodeof a solar cell on one end of the assembly with the exposed portion ofthe conductive core of the solar cell on the opposite end of theassembly.

In some embodiments, a counter-electrode comprises one or more layers ofany conductive material. For example, the counter-electrode comprises abead or strip of nickel, which may be coated with a layer of anotherconductive material such as aluminum, molybdenum, copper, steel, nickel,silver, gold, or an alloy thereof. In a preferred embodiment, thetransparent conductive oxide layer of a solar cell comprises zinc oxideand the counter-electrode comprises nickel, which may or may not becoated with another conductive metal such as, for example, aluminum. Inanother preferred embodiment, the transparent conductive oxide layer ofa solar cell comprises indium tin oxide and the counter-electrodecomprises silver, which may or may not be coated with another conductivematerial such as aluminum, molybdenum, copper, steel, nickel, gold, oran alloy thereof. In other embodiments, the counter-electrode is a thinstrip of nickel or silver epoxy.

In some embodiments counter-electrode is a conductive tape, or a contacttape with a conductive bonding. The conductive bonding may be attachedto the TCO layer, which can be made of material such as aluminum dopedzinc oxide, indium-zinc oxide, or indium-tin oxide, or other materials.In general, any conductive tape comprising an adhesive with a backingonto which is deposited a conductive material (e.g., silver, tin,nickel, copper, graphite, or aluminum) may be used as acounter-electrode. The bonding layer is preferably conductive andcompatible with zinc oxide and/or other materials used in the TCO layer.

Another aspect of the present invention provides a solar cell assemblycomprising a plurality of elongated solar cells. Each elongated solarcell in the plurality of elongated solar cells comprises (i) aconductive core configured as a first electrode, (ii) a semiconductorjunction circumferentially disposed on the conductive core, (iii) atransparent conductive oxide layer disposed on the semiconductorjunction, and (iv) an elongated counter-electrode disposed lengthwise ona first side of the solar cell and extending outward from thetransparent conductive oxide layer. On a second side of each solar cell,approximately opposite the counter-electrode, is a notch or otherdisruption extending through the semiconductor junction and thetransparent oxide layer, such that the conductive core of the solar cellis exposed. The solar cell assembly further comprises a plurality ofinternal reflectors. Each respective internal reflector in the pluralityof internal reflectors is configured between a corresponding first andsecond elongated solar cell in the plurality of elongated solar cellssuch that a portion of the solar light reflected from the respectiveinternal reflector is reflected onto the corresponding first and secondelongated solar cell. Each internal reflector in the plurality ofinternal reflectors is preferably conductive, and contacts theconductive core of the corresponding first electrode and thecounter-electrode of the corresponding second electrode. To complete thecircuit, a lead or wire may connect the counter electrode of a solarcell on one end of the assembly with the exposed portion of theconductive core of the solar cell on the opposite end of the assembly.

In some embodiments, the elongated conductive core of each respectiveelongated solar cell in the plurality of elongated solar cells has afirst exposed terminal portion not covered by the semiconductor junctionand the transparent conductive oxide layer of the respective elongatedsolar cell. In such embodiments, the solar cell assembly furthercomprises a first plurality of counter-electrode collars. Eachrespective counter-electrode collar in the first plurality ofcounter-electrode collars is wrapped around the transparent conductiveoxide layer of a corresponding elongated solar cell in the plurality ofelongated solar cells, toward a first end of the elongated solar cell,such that the respective counter-electrode collar is in electricalcommunication with the metal counter-electrode that lies in the firstgroove of the elongated solar cell. In such embodiments, the solar cellassembly further comprises a first plurality of electrical contacts.Each electrical contact in the first plurality of electrical contactselectrically connects the counter-electrode collar of a first elongatedsolar cell in the plurality of elongated solar cells, toward the firstend of the elongated solar cell, with the exposed first terminal portionof the elongated conductive core of a second elongated solar cell insaid plurality of elongated solar cells. In some embodiments, anelectrical contact in the first plurality of electrical contacts is madeof a conductive tape (e.g., a conductive tape that comprises a silver,nickel, tin, gold, copper, graphite, or aluminum deposit). In someembodiments, an electrical contact and the counter-electrode collar ofthe first elongated solar cell to which the electrical contact iselectrically connected are a single piece patterned such that thecounter-electrode collar wraps around the first elongated solar cell. Insome embodiments, a first elongated solar cell and a second elongatedsolar cell in the plurality of elongated solar cells are electricallyconnected in series.

In some embodiments, the first face and the second face of the planararray is each coated with a first layer of transparent insulator that isapplicable in atomized form. In some embodiments, the first face and thesecond face of the planar array is each coated with a second layer oftransparent insulator, over the first layer of transparent insulator(e.g., ethylene vinyl acetate), that is applicable in liquid or solidform.

In some embodiments, the semiconductor junction of an elongated solarcell in said plurality of elongated solar cells is a homojunction, aheterojunction, a heteroface junction, a buried homojunction, or a p-i-njunction. In some embodiments, there is an intrinsic layer disposedbetween the semiconductor junction and the transparent conductive oxidein an elongated solar cell in the plurality of elongated solar cells. Insome embodiments, the intrinsic layer is formed by an undopedtransparent oxide. In some embodiments, the intrinsic layer is made ofundoped zinc oxide, metal oxide or any transparent material that ishighly insulating.

In some embodiments, the elongated conductive core is made of aluminum,titanium, molybdenum, steel, nickel, silver, gold, an alloy thereof, orany combination thereof. In some embodiments, the transparent conductiveoxide layer of an elongated solar cell in the plurality of elongatedsolar cells is made of tin oxide SnO_(x), with or without fluorinedoping, indium-tin oxide (ITO), indium-zinc oxide, doped zinc oxide(e.g., aluminum doped zinc oxide) or a combination thereof. In apreferred embodiment, the conductive core is substantiallynon-conductive, and is surrounded by a conductive layer, such as a layerof molybdenum.

In some embodiments, solar cells are electrically connected to eachother in series by arranging the solar cells such that counter electrodeof each solar cell contacts the conductive core of an adjacent solarcell, e.g., through a notch or other disruption (e.g., a notch, scratch,break, void, channel, cavity or other disruption, generally referred toherein as a “notch”) formed in the outer layers to expose the conductivecore. An electrical lead or wire can be used to complete a circuit byproviding an electrical connection between a counter electrode on oneend of the solar cell assembly and an exposed conductive core of a solarcell on the opposite end of the solar cell assembly.

In some embodiments, the elongated conductive core of all or a portionof the elongated solar cells in the plurality of elongated solar cellsis hollowed. In some such embodiments, air or helium is blown throughall or a portion of the elongated solar cells in the plurality ofelongated solar cells. In some embodiments, an elongated solar cell inthe plurality of elongated solar cells is rod-shaped. In someembodiments, an elongated conductive core of an elongated solar cell inthe plurality of elongated solar cells is metal tubing.

Still another aspect of the invention provides a solar cell assemblycomprising a plurality of elongated solar cells, each elongated solarcell in the plurality of elongated solar cells including (i) anelongated conductive core configured as a first electrode, (ii) asemiconductor junction circumferentially disposed on the elongatedconductive core, and (iii) a transparent conductive oxide layer disposedon the semiconductor junction. In this aspect of the invention,elongated solar cells in the plurality of elongated solar cells aregeometrically arranged in a parallel or a near parallel manner therebyforming a planar array having a first face and a second face. The solarcell assembly further comprises a plurality of metal counter-electrodes.Each respective elongated solar cell in the plurality of elongated solarcells is bound to a first and second corresponding metalcounter-electrode in the plurality of metal counter-electrodes such thatthe first corresponding metal counter-electrode lies in a first groovethat runs lengthwise on the respective elongated solar cell and thesecond corresponding metal counter-electrode lies in a second groovethat runs lengthwise on the respective elongated solar cell. The solarcell assembly further comprises a plurality of internal reflectors. Eachrespective internal reflector in the plurality of internal reflectors isconfigured between a corresponding first and second elongated solar cellin the plurality of elongated solar cells such that a portion of thesolar light reflected from the respective internal reflector isreflected onto the corresponding first and second elongated solar cell.The solar cell assembly further comprises a transparent electricallyinsulating substrate that covers all or a portion of the first face ofthe planar array and a transparent insulating covering disposed on thesecond face of the planar array, thereby encasing the plurality ofelongated solar cells between the transparent insulating covering andthe transparent electrically insulating substrate.

Still another aspect of the invention provides a solar cell assemblycomprising a plurality of elongated solar cells. Each elongated solarcell in the plurality of elongated solar cells comprises (i) anelongated conductive core configured as a first electrode, (ii) asemiconductor junction circumferentially disposed on the elongatedconductive core; (iii) an intrinsic layer circumferentially disposed onthe semiconductor junction; and (iv) a transparent conductive oxidelayer disposed on the intrinsic layer. Elongated solar cells in theplurality of elongated solar cells are geometrically arranged in aparallel or a near parallel manner thereby forming a planar array havinga first face and a second face. The solar cell assembly furthercomprises a plurality of metal counter-electrodes. Each respectiveelongated solar cell in the plurality of elongated solar cells is boundto a first corresponding metal counter-electrode and a secondcorresponding metal counter-electrode in the plurality of metalcounter-electrodes such that (i) the first corresponding metalcounter-electrode lies in a first groove that runs lengthwise on therespective elongated solar cell and (ii) the second corresponding metalcounter-electrode lies in a second groove that runs lengthwise on therespective elongated solar cell. The solar cell assembly furthercomprises a plurality of internal reflectors. Each respective internalreflector in the plurality of internal reflectors is configured betweena corresponding first and second elongated solar cell in the pluralityof elongated solar cells such that a portion of the solar lightreflected from the respective internal reflector is reflected onto thecorresponding first and second elongated solar cell. The solar cellassembly further comprises a transparent electrically insulatingsubstrate that covers all or a portion of the first face of the planararray and a transparent insulating covering disposed on the second faceof the planar array, thereby encasing the plurality of elongated solarcells between the transparent insulating covering and the transparentelectrically insulating substrate.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates interconnected solar cells in accordance with theprior art.

FIG. 2 illustrates a spherical solar cell including a p-type inner layerand an n-type outer layer in accordance with the prior art.

FIG. 3A illustrates a tubular photovoltaic element comprising acylindrical silicon tube of n-type conductivity that has been subjectedto diffusion of boron into its outer surface to form an outerp-conductivity type region and thus a tubular solar cell in accordancewith the prior art.

FIG. 3B is a cross-sectional view of an elongated solar cell inaccordance with the prior art.

FIG. 3C is a cross-sectional view of a solar cell assembly in which aplurality of elongated solar cells are affixed to an electricallyconductive substrate in accordance with the prior art.

FIG. 3D is a cross-sectional view of a solar cell assembly disposed adistance away from a reflecting wall in accordance with the prior art.

FIG. 4A is a cross-sectional view of elongated solar cells electricallyarranged in series and geometrically arranged in a parallel or nearparallel manner on counter-electrodes that contact a substrate in orderto form a bifacial assembly, in accordance with an embodiment of thepresent invention.

FIG. 4B is a cross-sectional view taken about line 4B-4B of FIG. 4Adepicting the serial electrical arrangement of tubular solar cells in abifacial assembly in accordance with an embodiment of the presentinvention.

FIG. 4C is a blow-up perspective view of region 4C of FIG. 4B,illustrating various layers in elongated solar cells in accordance withone embodiment of the present invention.

FIG. 4D is a cross-sectional view of an elongated solar cell taken aboutline 4D-4D of FIG. 4B, in accordance with an embodiment of the presentinvention.

FIG. 4E is a cross-sectional view taken about line 4B-4B of FIG. 4A thatdepicts the serial arrangement of tubular solar cells in a bifacialassembly in accordance with an alternative embodiment of the presentinvention.

FIG. 4F is a cross-sectional view of an elongated solar cell taken aboutline 4F-4F of FIG. 4E, in accordance with an embodiment of the presentinvention.

FIGS. 5A-5D depict semiconductor junctions that are used in variouselongated solar cells in various embodiments of the present invention.

FIG. 6A is a cross-sectional view of elongated solar cells electricallyarranged in series in a bifacial assembly where counter-electrodes forminterfaces between solar cell pairs, in accordance with anotherembodiment of the present invention.

FIG. 6B is a cross-sectional view taken about line 6B-6B of FIG. 6A thatdepicts the serial arrangement of tubular solar cells in a bifacialassembly in accordance with an embodiment of the present invention.

FIG. 6C is a cross-sectional view of an elongated solar cell taken aboutline 6C-6C of FIG. 6B, in accordance with an embodiment of the presentinvention.

FIG. 7A is a cross-sectional view of elongated solar cells electricallyarranged in series in a bifacial assembly where counter-electrodes abutindividual solar cells, in accordance with another embodiment of thepresent invention.

FIG. 7B is a cross-sectional view taken about line 7B-7B of FIG. 7A thatdepicts the serial arrangement of tubular solar cells in a bifacialassembly in accordance with an embodiment of the present invention.

FIG. 8 is a cross-sectional view of elongated solar cells electricallyarranged in series in a bifacial assembly where counter-electrodes abutindividual solar cells and the outer TCO is cut, in accordance withanother embodiment of the present invention.

FIG. 9 is a cross-sectional view of elongated solar cells electricallyarranged in series in a bifacial assembly in which the inner metalelectrode is hollowed, in accordance with an embodiment of the presentinvention.

FIG. 10 is a cross-sectional view of elongated solar cells electricallyarranged in series in a bifacial assembly in which a groove pierces thecounter-electrodes, transparent conducting oxide layer, and junctionlayers of the solar cells, in accordance with an embodiment of thepresent invention.

FIG. 11 illustrates how the solar cell assemblies of the presentinvention can be used in conjunction with one type of staticconcentrator.

FIG. 12 illustrates how the solar cell assemblies of the presentinvention can be used in conjunction with another type of staticconcentrator.

FIG. 13 illustrates a solar cell made by a roll method in accordancewith an embodiment of the present invention.

FIG. 14 illustrates a perspective view of a solar cell architecture inaccordance with an embodiment of the present invention in whichreflectors are used to increase efficiency.

FIG. 15 illustrates a perspective view of a solar cell architecture inaccordance with an embodiment of the present invention in which anelectrode connects adjacent solar cells in series.

FIG. 16 illustrates a perspective view of a solar cell architecture inaccordance with an embodiment of the present invention in whichelectrodes connect adjacent solar cells in series.

FIG. 17 illustrates a cross-sectional view of a solar cell architecturein accordance with an embodiment of the present invention.

FIG. 18 illustrates a perspective view of an elongated solar cellarchitecture with protruding electrode attachments, in accordance withan embodiment of the present invention.

FIG. 19 illustrates a perspective view of a solar cell architecture inaccordance with an embodiment of the present invention.

FIG. 20 illustrates a perspective view of a solar cell architecture andtwo circuit board capping modules, in accordance with an embodiment ofthe present invention.

FIG. 21 illustrates a perspective view of a solar cell architecturewhose electrodes connect adjacent solar cells in series through circuitboard capping modules, in accordance with an embodiment of the presentinvention.

FIG. 22 illustrates a perspective view of a capped solar cellarchitecture contained in a support frame, in accordance with anembodiment of the present invention.

FIG. 23 illustrates a perspective view of a capped and sealed solar cellarchitecture contained in a support frame, in accordance with anembodiment of the present invention.

FIG. 24A illustrates light reflection on a specular surface.

FIG. 24B illustrates light reflection on a diffuse surface.

FIG. 24C illustrates light reflection on a Lambertian surface.

FIG. 25A illustrates a circle and an involute of the circle.

FIG. 25B illustrates a cross sectional view of a solar cell architecturein accordance with an embodiment of the present invention.

FIG. 26A is a cross-sectional view of elongated solar cells electricallyarranged in series in a bifacial assembly where elongatedcounter-electrodes extend from each solar cell and interface with theconductive core of the adjacent solar cell.

FIG. 26B is a close up of the solar cells of FIG. 26A

FIG. 26C is a cross sectional view of the solar cell assembly of FIG.26A, taken through line 26C.

FIG. 27A is a cross-sectional view of elongated solar cells of FIG. 26A,electrically arranged in series in a bifacial assembly where internalreflectors are disposed between adjacent solar cells and contact theconductive core and the counter-electrode of the adjacent solar cell.

FIG. 27B is a close-up view of the internal reflector and solar cells ofFIG. 27B.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings. Dimensions are not drawn to scale.

5. DETAILED DESCRIPTION

Disclosed herein are solar cell assemblies for converting solar energyinto electrical energy and more particularly to improved solar cells andsolar cell arrays. The solar cells of the present invention have a wireshape and are arranged in parallel but are electrically connected inseries.

5.1 Basic Structure

The present invention provides a solar cell assembly 400 in whichelongated solar cells 402, shown in cross-section in FIG. 4A, serve toabsorb light. A conductive core (elongated conductive core) 404 servesas the first electrode in the assembly and a transparent conductiveoxide (TCO) 412 on the exterior surface of each solar cell serves as thecounter-electrode.

In general, conductive core 404 is made out of any material such that itcan support the photovoltaic current generated by the solar cell withnegligible resistive losses. In some embodiments, conductive core 404 iscomposed of any conductive material, such as aluminum, molybdenum,titanium, steel, nickel, silver, gold, or an alloy thereof. In someembodiments, conductive core 404 is made out of a metal-, graphite-,carbon black-, or superconductive carbon black-filled oxide, epoxy,glass, or plastic. In some embodiments, conductive core 404 is made of aconductive plastic. As defined herein, a conductive plastic is one that,through compounding techniques, contains conductive fillers which, inturn, impart their conductive properties to the plastics system. Theconductive plastics used in the present invention to form conductivecore 404 contain fillers that form sufficient conductivecurrent-carrying paths through the plastic matrix to support thephotovoltaic current generated by solar cell with negligible resistivelosses. The plastic matrix of the conductive plastic is typicallyinsulating, but the composite produced exhibits the conductiveproperties of the filler.

A semiconductor junction 410 is formed around conductive core 404.Semiconductor junction 410 is any photovoltaic homojunction,heterojunction, heteroface junction, burried homojunction, or p-i-njunction having an absorber layer that is a direct band-gap absorber(e.g., crystalline silicon) or an indirect band-gap absorber (e.g.,amorphous silicon). Such junctions are described in Chapter 1 of Bube,Photovoltaic Materials, 1998, Imperial College Press, London, which ishereby incorporated by reference in its entirety. Details of exemplarytypes of semiconductors junctions 410 in accordance with the presentinvention are disclosed in Section 5.2, below. In addition to theexemplary junctions disclosed in Section 5.2, below, junctions 410 canbe multijunctions in which light traverses into the core of junction 410through multiple junctions that, preferably, have successfully smallerbandgaps.

Optionally, there is a thin intrinsic layer (i-layer) 415 betweensemiconductor junction 410 and an outer transparent conductive oxide(TCO) layer 412. The i-layer 415 can be formed using any undopedtransparent oxide including, but not limited to, zinc oxide, metaloxide, or any transparent material that is highly insulating.

The transparent conductive oxide (TCO) layer 412 is built up on top ofthe semiconductor junction layers 410 thereby completing the circuit. Asnoted above, in some embodiments, there is a thin i-layer coating thesemiconductor junction 410. In such embodiments, TCO layer 412 is builton top of the i-layer. In some embodiments, TCO layer 412 is made of tinoxide SnO_(x) (with or without fluorine doping), indium-tin oxide (ITO),doped zinc oxide (e.g., aluminum doped zinc oxide), indium-zinc oxide orany combination thereof. In some embodiments, TCO layer 412 is eitherp-doped or n-doped. For example, in embodiments where the outersemiconductor layer of junction 410 is p-doped, TCO layer 412 can bep-doped. Likewise, in embodiments where the outer semiconductor layer ofjunction 410 is n-doped, TCO layer 412 can be n-doped. In general, TCOlayer 412 is preferably made of a material that has very low resistance,suitable optical transmission properties (e.g., greater than 90%), and adeposition temperature that will not damage underlying layers ofsemiconductor junction 410 and/or optional i-layer 415. In someembodiments, TCO 412 is an electrically conductive polymer material suchas a conductive polytiophene, a conductive polyaniline, a conductivepolypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of anyof the foregoing. In some embodiments, TCO comprises more than onelayer, including a first layer comprising tin oxide SnO_(x) (with orwithout fluorine doping), indium-tin oxide (ITO), indium-zinc oxide,doped zinc oxide (e.g., aluminum doped zinc oxide) or a combinationthereof and a second layer comprising a conductive polytiophene, aconductive polyaniline, a conductive polypyrrole, a PSS-doped PEDOT(e.g., Bayrton), or a derivative of any of the foregoing. Additionalsuitable materials that can be used to form TCO layer are disclosed inUnited States Patent publication 2004/0187917A1 to Pichler, which ishereby incorporated by reference in its entirety.

Rod-shaped (elongated) solar cells 402 are lined up multiply inparallel. The entire assembly is sealed between electrically resistanttransparent substrate 406 and a covering 422 using a sealant such asethylene vinyl acetate or silicone. Covering 422 is generally made fromthe same materials as substrate 406. Suitable materials for covering 422and substrate 406 include, but are not limited to, glass or polyvinylfluoride products such as TEDLAR and TEFZEL (DuPont, Wilmington, Del.).

FIG. 4B provides a cross-sectional view with respect to line 4B-4B ofFIG. 4A. As can be seen with FIGS. 4A and 4B, each elongated cell 402has a length that is great compared to the diameter d of itscross-section. An advantage of the architecture shown in FIG. 4A is thatthere is no front side contact that shades solar cells 402. Such a frontside contact is found in known devices (e.g., elements 10 of FIG. 3).Another advantage of the architecture shown in FIG. 4A is that elongatedcells 402 are electrically connected in series rather than in parallel.In such a series configuration, the voltage of each elongated cell 402is summed. This serves to increase the voltage across the system,thereby keeping the current down, relative to comparable parallelarchitectures, and minimizes resistive losses. A serial electricalarrangement is maintained by arranging all or a portion of the elongatedsolar cells 402 such that they do not touch each other, as illustratedin FIGS. 4A and 4B. The separation distance between solar cells 402 isany distance that prevents electrical contact between solar cells 402.For instance, in some embodiments, the distance between adjacent solarcells 402 is 0.1 micron or greater, 0.5 microns or greater, or between 1and 5 microns.

Another advantage of the architecture shown in FIG. 4A is that theresistance loss across the system is low. This is because each electrodecomponent of the circuit is made of highly conductive material. Forexample, as noted above, conductive core 404 of each solar cell 402 ismade of a conductive metal. Furthermore, each conductive core 404 has adiameter that is large enough to carry current without an appreciablecurrent loss due to resistance. While larger conductive cores 404 ensurelow resistance, TCO layers encompassing such larger conductive cores 404must carry current further to contacts (counter-electrode strip) 420.Thus, there is an upper bound on the size of conductive cores 404. Inview of these and other considerations, diameter d is between 0.5millimeters (mm) and 20 mm in some embodiments of the present invention.Thus, conductive cores 404 are sized so that they are large enough tocarry a current without appreciable resistive losses, yet small enoughto allow TCO 412 to efficiently deliver current to leads 420. With thisbalanced design, resistive loss is minimized and an efficient solar cellassembly 400 is realized.

The advantageous low resistance nature of the architecture illustratedin FIG. 4A is also facilitated by the highly conductive properties ofleads 420. In some embodiments, for example, leads 420 are composed of aconductive epoxy (e.g., silver epoxy) or conductive ink and the like.

There are a number of different ways in which elongated cells 402 can bepackaged in order to form solar cell assembly 400. For example, in oneembodiment, leads 420 are formed by depositing a thin metallic layer onsubstrate 406 and then patterning the layer into a series of parallelstrips, where each strip runs the length of a solar cell 402. Then,elongated solar cells 402 are affixed to substrate 406 by leads 420using a conductive epoxy. In some embodiments, leads 420 are formeddirectly on solar cells 402 and solar cells 402 are not affixed tosubstrate 406. In such embodiments, there are at least two differentways in which elongated solar cells 402 can be packaged to form solarcell assembly 400. In a first approach, elongated solar cells 402,having leads 420 as illustrated in FIG. 4A, rest on substrate 406 butare not affixed to the substrate. In a second approach, elongated solarcells 402, having leads 420 as illustrated in FIG. 4A, do not contactsubstrate 406. This second approach is not illustrated. In this secondapproach, a layer of ethylene vinyl acetate or some other suitabletransparent material separates contacts 420 from substrate 406.

Still another advantage of the architecture illustrated in FIG. 4A isthat the path length through the absorber layer (e.g., layer 502, 510,520, or 540 of FIG. 5) of semiconductor junction 410 is, on average,longer than the path length through of the same type of absorber layerhaving the same width but in a planar configuration. Thus, the elongatedarchitecture illustrated in FIG. 4A allows for the design of thinnerabsorption layers relative to analogous planar solar cell counterparts.In the elongated architecture, the thinner absorption layer absorbs thelight because of the increased path length through the layer. Becausethe absorption layer is thinner relative to comparable planar solarcells, there is less resistance and, hence, an overall increase inefficiency in the cell relative to analogous planar solar cells.Additional advantages of having a thinner absorption layer that stillabsorbs sufficient amounts of light is that such absorption layersrequire less material and are thus cheaper. Furthermore, thinnerabsorption layers are faster to make, thereby further loweringproduction costs.

Another advantage of elongated solar cells 402 illustrated in FIG. 4A isthat they have a relatively small surface area, relative to comparableplanar solar cells, and they possess radial symmetry. Each of theseproperties allow for the controlled deposition of doped semiconductorlayers necessary to form semiconductor junction 410. The smaller surfacearea, relative to conventional flat panel solar cells, means that it iseasier to present a uniform vapor across the surface during depositionof the layers that form semiconductor junction 410. The radial symmetrycan be exploited during the manufacture of the cells in order to ensureuniform composition (e.g., uniform material composition, uniform dopantconcentration, etc.) and/or uniform thickness of individual layers ofsemiconductor junction 410. For example, the conductive core 404 uponwhich layers are deposited to make solar cells 402 can be rotated alongits longitudinal axis during such deposition in order to ensure uniformmaterial composition and/or uniform thickness.

The cross-sectional shape of solar cells 402 is generally circular inFIG. 4B. In other embodiments, solar cell 402 bodies with aquadrilateral cross-section or an elliptical shaped cross-section andthe like are used. In fact, there is no limit on the cross-sectionalshape of solar cells 402 in the present invention, so long as the solarcells 402 maintain a general overall rod-like or wire-like shape inwhich their length is much larger than their diameter and they possesssome form of cross-sectional radial symmetry.

As illustrated in FIG. 4B, assembly 400 comprises many elongated solarcells 402 geometrically arranged in parallel fashion and electricallyconnected in series. For example, a first and second elongated solarcell (rod-shaped solar cell) 402 are electrically connected in series byan electrical contact 433 that connects the conductive core 404 (firstelectrode) of the first elongated solar cell 402 to the correspondingcounter-electrode strip 420 electrode strip of the second elongatedsolar cell. Thus, as illustrated in FIG. 4A, elongated solar cells 402are the basic unit that respectively forms the semiconductor layer 410,the TCO 412, and the metal conductive core 404 of the elongated solarcell 402. The elongated solar cells 402 are multiply arranged in a rowparallel or nearly parallel with respect to each other and rest uponindependent leads (counter-electrodes) 420 that are electricallyisolated from each other. Advantageously, in the configurationillustrated in FIG. 4A, elongated solar cells 402 can receive directlight either through substrate 406, covering 422, or both substrate 406and covering 422.

In some embodiments, not all elongated solar cells 402 in assembly 400are electrically arranged in series. For example, in some embodiments,there are pairs of elongated solar cells 402 that are electricallyarranged in parallel. A first and second elongated solar cell can beelectrically connected in parallel, and are thereby paired, by using afirst electrical contact (e.g., an electrically conducting wire, etc.,not shown) that joins the conductive core 404 of a first elongated solarcell to the second elongated solar cell. To complete the parallelcircuit, the TCO 412 of the first elongated solar cell 402 iselectrically connected to the TCO 412 of the second elongated solar cell402 either by contacting the TCOs of the two elongated solar cellseither directly or through a second electrical contact (not shown). Thepairs of elongated solar cells are then electrically arranged in series.In some embodiments, three, four, five, six, seven, eight, nine, ten,eleven or more elongated solar cells 402 are electrically arranged inparallel. These parallel groups of elongated solar cells 402 are thenelectrically arranged in series.

In some embodiments, rather than packaging solar cells 402 between asubstrate 406 and cover 422 using a sealant such as ethyl vinyl acetate,solar cells 402 arranged in the same planar parallel configurationillustrated in FIGS. 4A and 4B are encased in a rigid transparent film.Suitable materials for such a rigid transparent film include, but arenot limited to, polyvinyl fluoride products such as Tedlar (DuPont,Wilmington, Del.).

FIG. 4C is an enlargement of region 4C of FIG. 4B in which a portion ofconductive core 404 and transparent conductive oxide (TCO) 412 have beencut away to illustrate the positional relationship betweencounter-electrode strip 420, elongated cell 402, and electricallyresistant transparent substrate 406. Furthermore FIG. 4C illustrates howelectrical contact 433 joins the conductive core 404 of one elongatedsolar cell 402 to the counter-electrode 420 of another solar cell 402.

One advantage of the configuration illustrated in FIG. 4 is thatelectrical contacts 433 that serially connect solar cells 402 togetheronly need to be placed on one end of assembly 400, as illustrated inFIG. 4B. Thus, referring to FIG. 4D, which is a cross-sectional view ofa elongated solar 402 cell taken about line 4D-4D of FIG. 4B, it ispossible to completely seal far-end 455 of solar cell 402 in the mannerillustrated. In some embodiments, the layers in this seal are identicalto the layers circumferentially disposed lengthwise on conductive core404, namely, in order of deposition on conductive core 404,semiconductor junction 410, optional thin intrinsic layer (i-layer) 415,and transparent conductive oxide (TCO) layer 412. In such embodiments,end 455 can receive sun light and therefore contribute to the electricalgenerating properties of the solar cell 402.

FIG. 4D also illustrates how the various layers deposited on conductivecore 404 are tapered at end 466 where electrical contacts 433 are found.For instance, a terminal portion of conductive core 404 is exposed, asillustrated in FIG. 4D. In other words, semiconductor junction 410,optional i-layer 415, and TCO 412 are stripped away from a terminalportion of conductive core 404. Furthermore, a terminal portion ofsemiconductor junction 410 is exposed as illustrated in FIG. 4D. Thatis, optional i-layer 415 and TCO 412 are stripped away from a terminalportion of semiconductor junction 410. Such a configuration isadvantageous because it prevents a short from developing between TCO 412and conductive core 404. In FIG. 4D, elongated solar cell 402 ispositioned on counter-electrode strip 420 which, in turn, is positionedonto electrically resistant transparent substrate 406. However, there isno requirement that counter-electrode strip 420 make contact withelectrically resistant transparent substrate 406. In fact, in someembodiments, elongated solar cells 402 and their corresponding electrodestrips 420 are sealed between electrically resistant transparentsubstrate 406 and covering 422 in such a manner that they do not contactsubstrate 406 and covering 422. In such embodiments, elongated solarcells 402 and corresponding electrode strips 420 are fixedly held inplace by a sealant such as ethyl vinyl acetate.

FIG. 4D further provides a perspective view of electrical contacts 433that serially connect elongated solar cells 402. For instance, a firstelectrical contact 433-1 electrically interfaces with counter-electrode420 whereas a second electrical contact 433-2 electrically interfaceswith conductive core 404 (the first electrode of elongated solar cell402). First electrical contact 433-1 serially connects thecounter-electrode of elongated solar cell 402 to the conductive core 404of another elongated solar cell 402 in assembly 400. Second electricalcontact 433-2 serially connects the conductive core 404 of elongatedsolar cell 402 to the counter-electrode 420 of another elongated solarcell 402 in assembly 400.

FIG. 4E provides a cross-sectional view with respect to line 4B-4B ofFIG. 4A in accordance with another embodiment of the present invention.FIG. 4E is similar to FIG. 4B. However, in FIG. 4E, elongated solarcells 402 facing end 455 are not sealed as they are in FIG. 4B and FIG.4D. Thus, the ends of elongated solar cells 402 facing end 455 cannotcontribute to the photovoltaic potential of solar cell 402. However, theembodiment illustrated in FIG. 4E has the advantage of being easier tomake than the embodiment illustrated in FIGS. 4B and 4D. Furthermore, inmany instances, the loss of contribution to the photovoltaic potentialfrom end 455 is negligible because the surface area of such ends is sosmall. FIG. 4F is a cross-sectional view of a elongated solar 402 celltaken about line 4F-4F of FIG. 4E which further illustrates theconfiguration of end 455 of elongated solar cell 402 in accordance withthe embodiment of the invention illustrated in FIG. 4E.

FIG. 6 illustrates a solar cell assembly 600 in accordance with thepresent invention. Specifically, FIG. 6A is a cross-sectional view ofrod-shaped (elongated) solar cells 402 electrically arranged in seriesin a bifacial assembly 600 where counter-electrodes 420 form interfacesbetween solar cell pairs 402. As illustrated in FIG. 6A, solar cellassembly 600 comprises a plurality of elongated solar cells 402. Thereis no limit to the number of solar cells 402 in this plurality (e.g.,1000 or more, 10,000 or more, between 5,000 and one million solar cells402, etc.). As in the embodiment of the invention illustrated in FIG. 4and described above, each elongated solar cell 402 comprises aconductive core 404 with a semiconductor junction 410 circumferentiallydisposed on the conductive core. A transparent conductive oxide layer412 circumferentially disposed on the semiconductor junction 410completes the circuit.

As illustrated in FIGS. 6A and 6B, the plurality of elongated solarcells 402 are geometrically arranged in a parallel or near parallelmanner as a plurality of solar cell pairs so as to form a planar arrayhaving a first face (on side 633 of assembly 600 as illustrated in FIG.6A) and a second face (on side 655 of assembly 600 as illustrated inFIG. 6A). Solar cells 402 in a pair of solar cells do not touch thesolar cells 402 in an adjacent pair of solar cells. However, in theembodiment illustrated in FIG. 6, solar cells 402 within a given pair ofsolar cells are in electrical contact with each other through theircommon counter-electrode 420. Accordingly, assembly 600 comprises aplurality of metal counter-electrodes 420. Each respective metalcounter-electrode in the plurality of metal counter-electrodes joinstogether, lengthwise, elongated solar cells 402 in a corresponding solarcell pair in the plurality of solar cell pairs. As such, elongated solarcells 402 in a solar cell pair are electrically arranged in parallel,not series.

In some embodiments there is a first groove 677-1 and a second groove677-2 that each runs lengthwise on opposing sides of solar cell 402. InFIG. 6A, some but not all grooves 677 are labeled. In some embodiments,the counter-electrode 420 of each pair of solar cells 402 is fittedbetween opposing grooves 677 in the solar cell pair in the mannerillustrated in FIG. 6A. The present invention encompasses grooves 677that have a broad range of depths and shape characteristics and is by nomeans limited to the shape of the grooves 677 illustrated in FIG. 6A. Ingeneral, any type of groove 677 that runs along the long axis of a firstsolar cell 402 in a solar cell pair and that can accommodate all or partof counter-electrode 420 in a pairwise fashion together with an opposinggroove on the second solar cell 402 in the solar cell pair is within thescope of the present invention.

As illustrated in FIG. 6A, a transparent electrically insulatingsubstrate 406 covers all or a portion of face 655 of the planar array ofsolar cells. In some embodiments, solar cells 402 touch substrate 406.In some embodiments, solar cells 402 do not touch substrate 406. Inembodiments in which solar cells 402 do not touch substrate 406, asealant such as ethyl vinyl acetate is used to seal substrate 406 ontosolar cells 402.

FIG. 6B provides a cross-sectional view with respect to line 6B-6B ofFIG. 6A. As can be seen in FIGS. 6A and 6B, each elongated solar cell402 has a length that is great compared to the diameter of itscross-section. Typically each solar cell 402 has a rod-like shape (e.g.,has a wire shape). Each solar cell pair is electrically connected toother solar cell pairs in series by arranging the solar cell pairs suchthat they do not touch each other, as illustrated in FIGS. 4A and 4B.The separation distance between solar cells pairs is any distance thatprevents electrical contact between the cells. For instance, in someembodiments, the distance between adjacent solar cell pairs is 0.1micron or greater, 0.5 microns or greater, or between 1 and 5 microns.Serial electrical contact between solar cell pairs is made by electricalcontacts 690 that electrically connect the conductive cores 404 of eachelongated solar cell in a one solar cell pair to the correspondingcounter-electrode 120 of a different solar cell pair as illustrated inFIG. 6B. FIG. 6B further illustrates a cutaway of conductive core 404and semiconductor junction 410 in one solar cell 402 to furtherillustrate the architecture of the solar cells.

Referring back to FIG. 6A, in some embodiments, solar cell assembly 600further comprises a transparent insulating covering 422 disposed on face633 of the planar array of solar cells 402, thereby encasing theplurality of elongated solar cells 402 between the transparentinsulating covering 422 and the transparent electrically insulatingsubstrate 406. In such embodiments, transparent insulating covering 422and the transparent insulating substrate 406 are bonded together by asealant such as ethyl vinyl acetate. Although not illustrated in FIGS.6A and 6B, in preferred embodiments, there is an intrinsic layercircumferentially disposed between the semiconductor junction 410 andTCO 412. In some embodiments, this intrinsic layer is formed by anundoped transparent oxide such as zinc oxide, metal oxide, oranytransparent metal that is highly insulating.

In some embodiments, the semiconductor junction 410 of solar cells 402in assembly 600 comprise an inner coaxial layer and an outer coaxiallayer, where the outer coaxial layer comprises a first conductivity typeand the inner coaxial layer comprises a second, opposite, conductivitytype. In some embodiments, the inner coaxial layer comprisescopper-indium-gallium-diselenide (CIGS) and the outer coaxial layercomprises CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments,conductive core 404 and/or electrical contacts 690 and/orcounter-electrodes 420 are made of aluminum, molybdenum, steel, nickel,silver, gold, or an alloy thereof. In some embodiments, transparentconductive oxide layer 412 is made of tin oxide SnO_(x), with or withoutfluorine doping, indium-tin oxide (ITO), indium-zinc oxide, doped zincoxide (e.g., aluminum doped zinc oxide) or a combination thereof. Insome embodiments, transparent insulating substrate 406 and transparentinsulating covering 422 comprise glass or Tedlar. Although not shown inFIG. 6, in some embodiments, conductive core 404 is hollowed as depictedin FIG. 9.

FIG. 6C illustrates a cross-sectional view of an elongated solar 402cell taken about line 6C-6C of FIG. 46. FIG. 6C illustrates how thevarious layers deposited on conductive core 404 are tapered at eitherend 687 or 688 (end 687 as illustrated in FIG. 6C). For instance, aterminal portion of conductive core 404 is exposed, as illustrated inFIG. 6C. In other words, semiconductor junction 410, an optional i-layer(not shown), and TCO 412 are stripped away from a terminal portion ofconductive core 404 at an end of the solar cell. Furthermore, a terminalportion of semiconductor junction 410 is exposed as illustrated in FIG.6C. That is, optional i-layer (not shown) and TCO 412 are stripped awayfrom the terminal portion of semiconductor junction 410 at an end of thesolar cell (end 687 in FIG. 6C). Such a configuration is advantageousbecause it prevents an electrical short from developing between TCO 412and conductive core 404. In FIG. 6C, elongated solar cell 402 ispositioned on electrically resistant transparent substrate 406. However,there is no requirement that elongated solar cell 402 make directcontact with electrically resistant transparent substrate 406. In fact,in some embodiments, elongated solar cells 402 are sealed betweenelectrically resistant transparent substrate 406 and covering 422 insuch a manner that they do not contact substrate 406 and covering 422.In such embodiments, elongated solar cells 402 are fixedly held in placeby a sealant such as ethyl vinyl acetate.

In some embodiments, not all elongated solar cell pairs in assembly 600are electrically arranged in series. For example, in some embodiments,two or more pairs of elongated solar cells are themselves paired suchthat all the elongated solar cells in the paired pairs are electricallyarranged in parallel. This can be accomplished by joining the conductivecore 404 of each of the solar cells by a common electrical contact(e.g., an electrically conducting wire, etc., not shown). To completethe parallel circuit, the TCO 412 of each of the elongated solar cell402 are electrically joined together either by direct contact or by theuse of a second electrical contact (not shown). The paired pairs ofelongated solar cells are then electrically arranged in series. In someembodiments, three, four, five, six, seven, eight, nine, ten, eleven ormore pairs of elongated solar cells are electrically arranged inparallel. These parallel groups of elongated solar cells 402 are thenelectrically arranged in series.

FIG. 7 illustrates solar cell assembly 700 in accordance with anotherembodiment of the present invention. Solar cell assembly 700 comprises aplurality of elongated solar cells 402. Each elongated solar cell 402 inthe plurality of elongated solar cells has a conductive core 404configured as a first electrode, a semiconductor junction 410circumferentially disposed on the conductive core 402 and a transparentconductive oxide layer 412 disposed on the semiconductor junction 410.The plurality of elongated solar cells 402 are geometrically arranged ina parallel or a near parallel manner thereby forming a planar arrayhaving a first face (facing side 733 of assembly 700) and a second face(facing side 766 of assembly 700). The plurality of elongated solarcells is arranged such that one or more elongated solar cells in theplurality of elongated solar cells do not contact adjacent elongatedsolar cells. In preferred embodiments, the plurality of elongated solarcells is arranged such that each of the elongated solar cells in theplurality of elongated solar cells does not directly contact (throughouter the TCO layer 412) adjacent elongated solar cells 402.

In some embodiments there is a first groove 777-1 and a second groove777-2 that each runs lengthwise on opposing sides of solar cell 402. InFIG. 7A, some but not all grooves 777 are labeled. In some embodiments,there is a counter-electrode 420 in one or both grooves of the solarcells. In the embodiment illustrated in FIG. 6A, there is acounter-electrode fitted lengthwise in both the first and second groovesof each solar cell in the plurality of solar cells. Such a configurationis advantageous because it reduces the pathlength of current drawn offof TCO 412. In other words, the maximum length that current must travelin TCO 412 before it reaches a counter-electrode 420 is a quarter of thecircumference of the TCO. By contrast, in configurations where there isonly a single counter-electrode 420 associated with a given solar cell402, the maximum length that current must travel in TCO 412 before itreaches a counter-electrode 420 is a full half of the circumference ofthe TCO. The present invention encompasses grooves 777 that have a broadrange of depths and shape characteristics and is by no means limited tothe shape of the grooves 777 illustrated in FIG. 7A. In general, anygroove shape 777 that runs along the long axis of a solar cell 402 andthat can accommodate all or part of counter-electrode 420 is within thescope of the present invention. For example, in some embodiments notillustrated by FIG. 7A, each groove 777 is patterned so that there is atight fit between the contours of the groove 777 and thecounter-electrode 420.

As illustrated in FIG. 7A, there are a plurality of metalcounter-electrodes 420, and each respective elongated solar cell 402 inthe plurality of elongated solar cells is bound to at least a firstcorresponding metal counter-electrode 420 in the plurality of metalcounter-electrodes such that the first metal counter-electrode lies in agroove 777 that runs lengthwise along the respective elongated solarcell. Furthermore, in the solar cell assembly illustrated in FIG. 7A,each respective elongated solar cell 402 is bound to a secondcorresponding metal counter-electrode 420 such that the second metalcounter-electrode lies in a second groove 777 that runs lengthwise alongthe respective elongated solar cell 402. As further illustrated in FIG.7A, the first groove 777 and the second groove 777 are on opposite orsubstantially opposite sides of the respective elongated solar cell 402and run along the long axis of the cell.

Further illustrated in FIG. 7A, is a transparent electrically insulatingsubstrate 406 that covers all or a portion of face 766 of the planararray. The plurality of elongated solar cells 402 are configured toreceive direct light from both face 733 and face 766 of the planararray. Solar cell assembly 700 further comprises a transparentinsulating covering 422 disposed on face 733 of the planar array,thereby encasing the plurality of elongated solar cells 402 between thetransparent insulating covering 422 and the transparent electricallyinsulating substrate 406.

FIG. 7B provides a cross-sectional view with respect to line 7B-7B ofFIG. 7A. Solar cell 402 are electrically connected to other in series byarranging the solar cells such that they do not touch each other, asillustrated in FIGS. 7A and 7B and by the use of electrical contacts asdescribed below in conjunction with FIG. 7B. The separation distancebetween solar cells 402 is any distance that prevents electrical contactbetween the TCO layers 412 of individual cells 402. For instance, insome embodiments, the distance between adjacent solar cells is 0.1micron or greater, 0.5 microns or greater, or between 1 and 5 microns.

Referring to FIG. 7B, serial electrical contact between solar cells 402is made by electrical contacts 788 that electrically connect the metalconductive core 404 of one elongated solar cell 402 to the correspondingcounter-electrodes 120 of a different solar cell 402 as illustrated inFIG. 7B. FIG. 7B further illustrates a cutaway of metal conductive core404 and semiconductor junction 410 in one solar cell 402 to furtherillustrate the architecture of the solar cells 402.

The solar cell assembly illustrated in FIG. 7 has several advantages.First, because of the positioning of counter-electrodes 420 and thetransparency of both substrate 406 and covering 422, there is almostzero percent shading in the assembly. For instance, the assembly canreceive direct sunlight from both face 733 and face 766. Second, inembodiments where a sealant such as ethyl vinyl acetate (EVA) is used tolaminate substrate 406 and covering 422 onto the plurality of solarcells, the structure is completely self-supporting. Still anotheradvantage of the assembly is that is easy to manufacture. Unlike solarcells such as that depicted in FIG. 3A, no complicated grid ortransparent conductive oxide on glass is needed. For example, toassemble a solar cell 402 and its corresponding counter-electrodes 420together to complete the circuit illustrated in FIG. 7A,counter-electrode 420, when it is in the form of a wire, can be coveredwith conductive epoxy and dropped in the groove 777 of solar cell 402and allowed to cure.

As illustrated in FIG. 7B, conductive core 404, junction 410, and TCO412 are flush with each other at end 789 of elongated solar cells 402.In contrast, at end 799 conductive core protrudes a bit with respect tojunction 410 and TCO 412 as illustrated. Junction 410 also protrudes abit at end 799 with respect to TCO 412. The protrusion of conductivecore 404 at end 799 means that the sides of a terminal portion of theconductive core 404 are exposed (e.g., not covered by junction 410 andTCO 412). The purpose of this configuration is to reduce the chances ofshorting counter-electrode 420 (or the epoxy used to mount thecounter-electrode in groove 777) with TCO 412. In some embodiments, allor a portion of the exposed surface area of counter-electrodes 420 areshielded with an electrically insulating material in order to reduce thechances of electrical shortening. For example, in some embodiments, theexposed surface area of counter-electrodes 420 in the boxed regions ofFIG. 7B is shielded with an electrically insulating material.

Still another advantage of the assembly illustrated in FIG. 7 is thatthe counter-electrode 420 can have much higher conductivity withoutshadowing. In other words, counter-electrode 420 can have a substantialcross-sectional size (e.g., 1 mm in diameter when solar cell 402 has a 6mm diameter). Thus, counter-electrode 420 can carry a significant amountof current so that the wires can be as long as possible, thus enablingthe fabrication of larger panels.

The series connections between solar cells 402 can be between pairs ofsolar cells 402 in the manner depicted in FIG. 7B. However, theinvention is not so limited. In some embodiments, two or more solarcells 402 are grouped together (e.g., electrically connected in aparallel fashion) to form a group of solar cells and then such groups ofsolar cells are serially connected to each other. Therefore, the serialconnections between solar cells can be between groups of solar cellswhere such groups have any number of solar cells 402 (e.g., 2, 3, 4, 5,6, etc.). However, FIG. 7B illustrates a preferred embodiment in whicheach contact 788 serially connects only a pair of solar cells 402.

In some embodiments, there is a series insulator that runs lengthwisebetween each solar cell 402. In one example, this series insulator is a0.001″ thick sheet of transparent insulating plastic. In other examplesthis series insulator is a sheet of transparent insulating plastichaving a thickness between 0.001″ and 0.005″. Alternatively, a roundinsulating clear plastic separator that runs lengthwise between solarcells 402 can be used to electrically isolate the solar cells 402.Advantageously, any light that does enter the small gap between solarcells 402 will be trapped and collected in the “double-divot” areaformed by facing grooves 777 of adjacent solar cells 402.

Yet another embodiment of solar cell assembly 700 is that there is noextra absorption loss from a TCO or a metal grid on one side of theassembly. Further, assembly 700 has the same performance or absorberarea exposed on both sides 733 and 766. This makes assembly 700symmetrical.

Still another advantage of assembly 700 is that all electrical contacts788 end at the same level (e.g., in the plane of line 7B-7B of FIG. 7A).As such, they are easier to connect and weld with very little substratearea wasted at the end. This simplifies construction of the solar cells402 while at the same time serves to increase the overall efficiency ofsolar cell assembly 700. This increase in efficiency arises because thewelds can be smaller. Smaller welds take up less of the electricallyresistant transparent substrate 406 surface area that is otherwiseoccupied by solar cells 402.

Although not illustrated in FIG. 7, in some embodiments in accordancewith FIG. 7, there is an intrinsic layer circumferentially disposedbetween the semiconductor junction 410 and the transparent conductiveoxide 412 in an elongated solar cell 402 in the plurality of elongatedsolar cells 402. This intrinsic layer can be made of an undopedtransparent oxide such as zinc oxide, metal oxide, or any transparentmetal that is highly insulating. In some embodiments, the semiconductorjunction 410 of solar cells 402 in assembly 700 comprise an innercoaxial layer and an outer coaxial layer where the outer coaxial layercomprises a first conductivity type and the inner coaxial layercomprises a second, opposite, conductivity type. In an exemplaryembodiment the inner coaxial layer comprisescopper-indium-gallium-diselenide (CIGS) whereas the outer coaxial layercomprises CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments notillustrated by FIG. 7, the conductive cores 404 in solar cells 402 arehollowed.

FIG. 8 illustrates a solar cell assembly 800 of the present inventionthat is identical to solar cell assembly 700 of the present inventionwith the exception that TCO 412 is interrupted by breaks 810 that runalong the long axis of solar cells 402 and cut completely through TCO412. In the embodiment illustrated in FIG. 8, there are two breaks 810that run the length of solar cell 402. The effect of such breaks 810 isthat they electrically isolate the two counter-electrodes 420 associatedwith each solar cell 402 in solar cell assembly 800. There are many waysin which breaks 800 can be made. For example, a laser or an HCl etch canbe used.

In some embodiments, not all elongated solar cells 402 in assembly 800are electrically arranged in series. For example, in some embodiments,there are pairs of elongated solar cells 402 that are electricallyarranged in parallel. A first and second elongated solar cell can beelectrically connected in parallel, and are thereby paired, by using afirst electrical contact (e.g., an electrically conducting wire, etc.,not shown) that joins the conductive core 404 of a first elongated solarcell to the second elongated solar cell. To complete the parallelcircuit, the TCO 412 of the first elongated solar cell 402 iselectrically connected to the TCO 412 of the second elongated solar cell402 either by contacting the TCOs of the two elongated solar cellseither directly or through a second electrical contact (not shown). Thepairs of elongated solar cells are then electrically arranged in series.In some embodiments, three, four, five, six, seven, eight, nine, ten,eleven or more elongated solar cells 402 are electrically arranged inparallel. These parallel groups of elongated solar cells 402 are thenelectrically arranged in series.

FIG. 9 illustrates a solar cell assembly 900 of the present invention inwhich conductive cores 402 are hollowed. In fact, conductive cores 402can be hollowed in any of the embodiments of the present invention. Oneadvantage of such a hollowed core 402 design is that it reduces theoverall weight of the solar cell assembly. Core 402 is hollowed whenthere is a channel that extends lengthwise through all or a portion ofcore 402. In some embodiments, conductive core 402 is metal tubing.

In some embodiments, not all elongated solar cells 402 in assembly 900are electrically arranged in series. For example, in some embodiments,there are pairs of elongated solar cells 402 that are electricallyarranged in parallel. A first and second elongated solar cell can beelectrically connected in parallel, and are thereby paired, by using afirst electrical contact (e.g., an electrically conducting wire, etc.,not shown) that joins the conductive core 404 of a first elongated solarcell to the second elongated solar cell. To complete the parallelcircuit, the TCO 412 of the first elongated solar cell 402 iselectrically connected to the TCO 412 of the second elongated solar cell402 either by contacting the TCOs of the two elongated solar cellseither directly or through a second electrical contact (not shown). Thepairs of elongated solar cells are then electrically arranged in series.In some embodiments, three, four, five, six, seven, eight, nine, ten,eleven or more elongated solar cells 402 are electrically arranged inparallel. These parallel groups of elongated solar cells 402 are thenelectrically arranged in series.

FIG. 10 illustrates a solar cell assembly 1000 of the present inventionin which counter-electrodes 420, TCOs 412, and junctions 410 arepierced, in the manner illustrated, in order to form two discretejunctions in parallel.

5.2 Exemplary Semiconductor Junctions

Referring to FIG. 5A, in one embodiment, semiconductor junction 410 is aheterojunction between an absorber layer 502, disposed on conductivecore 404, and a junction partner layer 504, disposed on absorber layer502. Layers 502 and 504 are composed of different semiconductors withdifferent band gaps and electron affinities such that junction partnerlayer 504 has a larger band gap than absorber layer 502. In someembodiments, absorber layer 502 is p-doped and junction partner layer504 is n-doped. In such embodiments, TCO layer 412 is n⁺-doped. Inalternative embodiments, absorber layer 502 is n-doped and junctionpartner layer 504 is p-doped. In such embodiments, TCO layer 412 isp⁺-doped. In some embodiments, the semiconductors listed in Pandey,Handbook of Semiconductor Electrodeposition, Marcel Dekker Inc., 1996,Appendix 5, hereby incorporated by reference in its entirety, are usedto form semiconductor junction 410.

5.2.1 Thin-Film Semiconductor Junctions Based on Copper IndiumDiselenide and Other Type I-III-VI Materials

Continuing to refer to FIG. 5A, in some embodiments, absorber layer 502is a group I-III-VI₂ compound such as copper indium di-selenide(CuInSe₂; also known as CIS). In some embodiments, absorber layer 502 isa group I-III-VI₂ ternary compound selected from the group consisting ofCdGeAs₂, ZnSnAs₂, CuInTe₂, AgInTe₂, CuInSe₂, CuGaTe₂, ZnGeAs₂, CdSnP₂,AgInSe₂, AgGaTe₂, CuInS₂, CdSiAs₂, ZnSnP₂, CdGeP₂, ZnSnAs₂, CuGaSe₂,AgGaSe₂, AgInS₂, ZnGeP₂, ZnSiAs₂, ZnSiP₂, CdSiP₂, or CuGaS₂ of eitherthe p-type or the n-type when such compound is known to exist.

In some embodiments, junction partner layer 504 is CdS, ZnS, ZnSe, orCdZnS. In one embodiment, absorber layer 502 is p-type CIS and junctionpartner layer 504 is n-type CdS, ZnS, ZnSe, or CdZnS. Such semiconductorjunctions 410 are described in Chapter 6 of Bube, PhotovoltaicMaterials, 1998, Imperial College Press, London, which is herebyincorporated by reference in its entirety. Such semiconductor junctions410 are described in Chapter 6 of Bube, Photovoltaic Materials, 1998,Imperial College Press, London, which is hereby incorporated byreference in its entirety.

In some embodiments, absorber layer 502 iscopper-indium-gallium-diselenide (CIGS). Such a layer is also known asCu(InGa)Se₂. In some embodiments, absorber layer 502 iscopper-indium-gallium-diselenide (CIGS) and junction partner layer 504is CdS, ZnS, ZnSe, or CdZnS. In some embodiments, absorber layer 502 isp-type CIGS and junction partner layer 504 is n-type CdS, ZnS, ZnSe, orCdZnS. Such semiconductor junctions 410 are described in Chapter 13 ofHandbook of Photovoltaic Science and Engineering, 2003, Luque andHegedus (eds.), Wiley & Sons, West Sussex, England, Chapter 12, which ishereby incorporated by reference in its entirety.

5.2.2 Semiconductor Junctions Based on Amorphous Silicon orPolycrystalline Silicon

In some embodiments, referring to FIG. 5B, semiconductor junction 410comprises amorphous silicon. In some embodiments this is an n/n typeheterojunction. For example, in some embodiments, layer 514 comprisesSnO₂(Sb), layer 512 comprises undoped amorphous silicon, and layer 510comprises n+ doped amorphous silicon.

In some embodiments, semiconductor junction 410 is a p-i-n typejunction. For example, in some embodiments, layer 514 is p⁺ dopedamorphous silicon, layer 512 is undoped amorphous silicon, and layer 510is n⁺ amorphous silicon. Such semiconductor junctions 410 are describedin Chapter 3 of Bube, Photovoltaic Materials, 1998, Imperial CollegePress, London, which is hereby incorporated by reference in itsentirety.

In some embodiments of the present invention, semiconductor junction 410is based upon thin-film polycrystalline. Referring to FIG. 5B, in oneexample in accordance with such embodiments, layer 510 is a p-dopedpolycrystalline silicon, layer 512 is depleted polycrystalline siliconand layer 514 is n-doped polycrystalline silicon. Such semiconductorjunctions are described in Green, Silicon Solar Cells: AdvancedPrinciples & Practice, Centre for Photovoltaic Devices and Systems,University of New South Wales, Sydney, 1995; and Bube, PhotovoltaicMaterials, 1998, Imperial College Press, London, pp. 57-66, which ishereby incorporated by reference in its entirety.

In some embodiments of the present invention, semiconductor junctions410 based upon p-type microcrystalline Si:H and microcrystalline Si:C:Hin an amorphous Si:H solar cell are used. Such semiconductor junctionsare described in Bube, Photovoltaic Materials, 1998, Imperial CollegePress, London, pp. 66-67, and the references cited therein, which ishereby incorporated by reference in its entirety.

5.2.3 Semiconductor Junctions Based on Gallium Arsenide and Other TypeIII-V Materials

In some embodiments, semiconductor junctions 410 are based upon galliumarsenide (GaAs) or other III-V materials such as InP, AlSb, and CdTe.GaAs is a direct-band gap material having a band gap of 1.43 eV and canabsorb 97% of AM1 radiation in a thickness of about two microns.Suitable type III-V junctions that can serve as semiconductor junctions410 of the present invention are described in Chapter 4 of Bube,Photovoltaic Materials, 1998, Imperial College Press, London, which ishereby incorporated by reference in its entirety.

Furthermore, in some embodiments semiconductor junction 410 is a hybridmultifunction solars cells such as a GaAs/Si mechanically stackedmultijunction as described by Gee and Virshup, 1988, 20^(th) IEEEPhotovoltaic Specialist Conference, IEEE Publishing, New York, p. 754,which is hereby incorporated by reference in its entirety, aGaAs/CuInSe₂ MSMJ four-terminal device, consisting of a GaAs thin filmtop cell and a ZnCdS/CuInSe₂ thin bottom cell described by Stanbery etal., 19^(th) IEEE Photovoltaic Specialist Conference, IEEE Publishing,New York, p. 280, and Kim et al., 20^(th) IEEE Photovoltaic SpecialistConference, IEEE Publishing, New York, p. 1487, each of which is herebyincorporated by reference in its entirety. Other hybrid multijunctionsolar cells are described in Bube, Photovoltaic Materials, 1998,Imperial College Press, London, pp. 131-132, which is herebyincorporated by reference in its entirety.

5.2.4 Semiconductor Junctions Based on Cadmium Telluride and Other TypeII-VI Materials

In some embodiments, semiconductor junctions 410 are based upon II-VIcompounds that can be prepared in either the n-type or the p-type form.Accordingly, in some embodiments, referring to FIG. 5C, semiconductorjunction 410 is a p-n heterojunction in which layers 520 and 540 are anycombination set forth in the following table or alloys thereof.

Layer 520 Layer 540 n-CdSe p-CdTe n-ZnCdS p-CdTe n-ZnSSe p-CdTe p-ZnTen-CdSe n-CdS p-CdTe n-CdS p-ZnTe p-ZnTe n-CdTe n-ZnSe p-CdTe n-ZnSep-ZnTe n-ZnS p-CdTe n-ZnS p-ZnTe

Methods for manufacturing semiconductor junctions 410 are based uponII-VI compounds are described in Chapter 4 of Bube, PhotovoltaicMaterials, 1998, Imperial College Press, London, which is herebyincorporated by reference in its entirety.

5.2.5 Semiconductor Junctions Based on Crystalline Silicon

While semiconductor junctions 410 that are made from thin semiconductorfilms are preferred, the invention is not so limited. In someembodiments semiconductor junctions 410 is based upon crystallinesilicon. For example, referring to FIG. 5D, in some embodiments,semiconductor junction 410 comprises a layer of p-type crystallinesilicon 540 and a layer of n-type crystalline silicon 550. Methods formanufacturing crystalline silicon semiconductor junctions 410 aredescribed in Chapter 2 of Bube, Photovoltaic Materials, 1998, ImperialCollege Press, London, which is hereby incorporated by reference in itsentirety.

5.3 Albedo Embodiments

The solar cell assemblies of the present invention are advantageousbecause they can collect light through either of their two faces.Accordingly, in some embodiments of the present invention, thesesbifacial solar cell assemblies (e.g., solar cell assembly 400, 600, 700,800, 900, etc.) are arranged in a reflective environment in whichsurfaces around the solar cell assembly have some amount of albedo.Albedo is a measure of reflectivity of a surface or body. It is theratio of electromagnetic radiation (EM radiation) reflected to theamount incident upon it. This fraction is usually expressed as apercentage from 0% to 100%. In some embodiments, surfaces in thevicinity of the solar cell assemblies of the present invention areprepared so that they have a high albedo by painting such surfaces areflective white color. In some embodiments, other materials that have ahigh albedo can be used. For example, the albedo of some materialsaround such solar cells approach or exceed ninety percent. See, forexample, Boer, 1977, Solar Energy 19, 525, which is hereby incorporatedby reference in its entirety. However, surfaces having any amount ofalbedo (e.g., five percent or more, ten percent or more, twenty percentor more) are within the scope of the present invention. In oneembodiment, the solar cells assemblies of the present invention arearranged in rows above a gravel surface, where the gravel has beenpainted white in order to improve the reflective properties of thegravel. In general, any Lambertian or diffuse reflector surface can beused to provide a high albedo surface.

In some embodiments, the bifacial solar cell assemblies of the presentinvention are placed in a manner such that one surface (e.g., face 633of solar cell assembly 600) is illuminated in a way similar to aconventional flat-panel solar cell panel. For example, it is installedfacing South (in the northern hemisphere) with an angle of inclinationthat is latitude dependent (e.g., in general is not very different fromthe latitude). The opposing surface of the bifacial solar cell assembly(e.g., face 655 of solar cell assembly 600) of the present inventionreceives a substantial amount of diffuse light reflected from the groundand neighboring walls in the vicinity of the solar cell assembly.

By way of example, in some embodiments of the present invention, thebifacial solar cell assemblies (panels) of the present invention have afirst and second face and are placed in rows facing South in theNorthern hemisphere (or facing North in the Southern hemisphere). Eachof the panels is placed some distance above the ground (e.g., 100 cmabove the ground). The East-West separation between the panels issomewhat dependent upon the overall dimensions of the panels. By way ofillustration only, panels having overall dimensions of about 106 cm×44cm are placed in the rows such that the East-West separation between thepanels is between 10 cm and 50 cm. In one specific example the East-Westseparation between the panels is 25 cm.

In some embodiments, the central point of the panels in the rows ofpanels is between 0.5 meters and 2.5 meters from the ground. In onespecific example, the central point of the panels is 1.55 meters fromthe ground. The North-South separation between the rows of panels isdependent on the dimensions of the panels. By way of illustration, inone specific example, in which the panels have overall dimensions ofabout 106 cm×44 cm, the North-South separation is 2.8 meters. In someembodiments, the North-South separation is between 0.5 meters and 5meters. In some embodiments, the North-South separation is between 1meter and 3 meters.

In some embodiments of the present invention, the panels in the rows areeach tilted with respect to the ground in order to maximize the totalamount of light received by the panels. There is some tradeoff betweenincreasing the amount of light received by one face versus the amount oflight received on the opposing face as a function of tilt angle.However, at certain tilt angles, the total amount of light received bythe panels, where total amount of light is defined as the sum of directlight received on the first and second face of the bifacial panel, ismaximized. In some embodiments, the panels in the rows of panels areeach tilted between five degrees and forty-five degrees from thehorizontal. In some embodiments, the panels of the present invention aretilted between fifteen degrees and forty degrees from the horizontal. Insome embodiments, the panels of the present invention are tilted betweentwenty-five degrees and thirty-five degrees from the horizontal. In onespecific embodiment, the panels of the present invention are tiltedthirty degrees from the horizontal.

In some embodiments, models for computing the amount of sunlightreceived by solar panels as put forth in Lorenzo et al., 1985, SolarCells 13, pp. 277-292, which is hereby incorporated by reference in itsentirety, are used to compute the optimum horizontal tilt and East-Westseparation of the solar panels in the rows of solar panels that areplaced in a reflective environment.

5.4 Dual Layer Core Embodiments

Embodiments of the present invention in which conductive core 404 of thesolar cells 402 of the present invention is made of a uniform conductivematerial have been disclosed. The invention is not limited to theseembodiments. In some embodiments, conductive core 404 in fact has aninner core and an outer conductive core. The outer conductive core iscircumferentially disposed on the inner core. In such embodiments, theinner core is typically nonconductive whereas the outer core isconductive. The inner core has an elongated shape consistent with otherembodiments of the present invention. For instance, in one embodiment,the inner core is made of glass fibers in the form of a wire. In someembodiments, the inner core is an electrically conductive nonmetallicmaterial. However, the present invention is not limited to embodimentsin which the inner core is electrically conductive because the outercore can function as the electrode. In some embodiments, the inner coreis tubing (e.g., plastic tubing).

In some embodiments, the inner core is made of a material such aspolybenzamidazole (e.g., Celazole®, available from Boedeker Plastics,Inc., Shiner, Texas). In some embodiments, the inner core is made ofpolymide (e.g., DuPont™ Vespel®, or DuPont Kapton®, Wilmington, Del.).In some embodiments, the inner core is made of polytetrafluoroethylene(PTFE) or polyetheretherketone (PEEK), each of which is available fromBoedeker Plastics, Inc. In some embodiments, the inner core is made ofpolyamide-imide (e.g., Torlon® PAI, Solvay Advanced Polymers,Alpharetta, Ga.).

In some embodiments, the inner core is made of a glass-based phenolic.Phenolic laminates are made by applying heat and pressure to layers ofpaper, canvas, linen or glass cloth impregnated with syntheticthermosetting resins. When heat and pressure are applied to the layers,a chemical reaction (polymerization) transforms the separate layers intoa single laminated material with a “set” shape that cannot be softenedagain. Therefore, these materials are called “thermosets.” A variety ofresin types and cloth materials can be used to manufacture thermosetlaminates with a range of mechanical, thermal, and electricalproperties. In some embodiments, the inner core is a phenoloic laminatehaving a NEMA grade of G-3, G-5, G-7, G-9, G-10 or G-11. Exemplaryphenolic laminates are available from Boedeker Plastics, Inc.

In some embodiments, the inner core is made of polystyrene. Examples ofpolystyrene include general purpose polystyrene and high impactpolystyrene as detailed in Marks' Standard Handbook for MechanicalEngineers, ninth edition, 1987, McGraw-Hill, Inc., p. 6-174, which ishereby incorporated by reference in its entirety. In still otherembodiments, inner core is made of cross-linked polystyrene. One exampleof cross-linked polystyrene is Rexolite® (available from San DiegoPlastics Inc., National City, Calif.). Rexolite is a thermoset, inparticular a rigid and translucent plastic produced by cross linkingpolystyrene with divinylbenzene.

In some embodiments, the inner core is a polyester wire (e.g., a Mylar®wire). Mylar® is available from DuPont Teijin Films (Wilmington, Del.).In still other embodiments, the inner core is made of Durastone®, whichis made by using polyester, vinylester, epoxid and modified epoxy resinscombined with glass fibers (Roechling Engineering Plastic Pte Ltd.(Singapore).

In still other embodiments, the inner core is made of polycarbonate.Such polycarbonates can have varying amounts of glass fibers (e.g., 10%,20%, 30%, or 40%) in order to adjust tensile strength, stiffness,compressive strength, as well as the thermal expansion coefficient ofthe material. Exemplary polycarbonates are Zelux® M and Zelux® W, whichare available from Boedeker Plastics, Inc.

In some embodiments, the inner core is made of polyethylene. In someembodiments, the inner core is made of low density polyethylene (LDPE),high density polyethylene (HDPE), or ultra high molecular weightpolyethylene (UHMW PE). Chemical properties of HDPE are described inMarks' Standard Handbook for Mechanical Engineers, ninth edition, 1987,McGraw-Hill, Inc., p. 6-173, which is hereby incorporated by referencein its entirety. In some embodiments, the inner core is made ofacrylonitrile-butadiene-styrene, polytetrfluoro-ethylene (Teflon),polymethacrylate (lucite or plexiglass), nylon 6,6, cellulose acetatebutyrate, cellulose acetate, rigid vinyl, plasticized vinyl, orpolypropylene. Chemical properties of these materials are described inMarks' Standard Handbook for Mechanical Engineers, ninth edition, 1987,McGraw-Hill, Inc., pp. 6-172 through 6-175, which is hereby incorporatedby reference in its entirety.

Additional exemplary materials that can be used to form the inner coreare found in Modern Plastics Encyclopedia, McGraw-Hill; ReinholdPlastics Applications Series, Reinhold Roff, Fibres, Plastics andRubbers, Butterworth; Lee and Neville, Epoxy Resins, McGraw-Hill;Bilmetyer, Textbook of Polymer Science, Interscience; Schmidt andMarlies, Principles of high polymer theory and practice, McGraw-Hill;Beadle (ed.), Plastics, Morgan-Grampiand, Ltd., 2 vols. 1970; Tobolskyand Mark (eds.), Polymer Science and Materials, Wiley, 1971; Glanville,The Plastics's Engineer's Data Book, Industrial Press, 1971; Mohr(editor and senior author), Oleesky, Shook, and Meyers, SPI Handbook ofTechnology and Engineering of Reinforced Plastics Composites, VanNostrand Reinhold, 1973, each of which is hereby incorporated byreference in its entirety.

In general, outer core is made out of any material that can support thephotovoltaic current generated by solar cell with negligible resistivelosses. In some embodiments, outer core is made of any conductive metal,such as aluminum, molybdenum, steel, nickel, silver, gold, or an alloythereof. In some embodiments, outer core is made out of a metal-,graphite-, carbon black-, or superconductive carbon black-filled oxide,epoxy, glass, or plastic. In some embodiments, outer core is made of aconductive plastic. In some embodiments, this conductive plastic isinherently conductive without any requirement for a filler. In someembodiments inner core is made out of a conductive material and outercore is made out of molybdenum.

In embodiments where an inner core and an outer core is present,semiconductor junction 410 and TCO 412 are stripped from the inner coreat a terminal end of the solar cell where an electrical contact seriallyjoins the solar cell to another solar cell. For example, in someembodiments, the semiconductor junction 410 and TCO are stripped in themanner illustrated in FIGS. 4D, 4F, 6B, 6C, and 7B.

5.5 Exemplary Dimensions

The present invention encompasses solar cell assemblies having anydimensions that fall within a broad range of dimensions. For example,referring to FIG. 4B, the present invention encompasses solar cellassemblies having a length l between 1 cm and 50,000 cm and a width wbetween 1 cm and 50,000 cm. In some embodiments, the solar cellassemblies have a length l between 10 cm and 1,000 cm and a width wbetween 10 cm and 1,000 cm. In some embodiments, the solar cellassemblies have a length l between 40 cm and 500 cm and a width wbetween 40 cm and 500 cm.

5.6 Solar Cells Manufactured Using a Roll Method or Having an Inner TCO

In some embodiments, copper-indium-gallium-diselenide (Cu(InGa)Se₂),referred to herein as CIGS, is used to make the absorber layer ofjunction 110. In such embodiments, conductive core 404 can be made ofmolybdenum. In some embodiments, core 404 comprises an inner core ofpolyimide and an outer core that is a thin film of molybdenum sputteredonto the polyimide core prior to CIGS deposition. On top of themolybdenum, the CIGS film, which absorbs the light, is evaporated.Cadmium sulfide (CdS) is then deposited on the CIGS in order to completesemiconductor junction 410. Optionally, a thin intrinsic layer (i-layer)is then deposited on the semiconductor junction 410. The i-layer can beformed using a material including but not limited to, zinc oxide, metaloxide or any transparent material that is highly insulating. Next, theTCO 412 is disposed on either the i-layer (when present) or thesemiconductor junction 410 (when the i-layer is not present). The TCOcan be made of a material such as aluminum doped zinc oxide (ZnO:Al),indium-zinc oxide, or indium-tin oxide.

ITN Energy Systems, Inc., Global Solar Energy, Inc., and the Instituteof Energy Conversion (IEC), have collaboratively developed technologyfor manufacturing CIGS photovoltaics on polyimide substrates using aroll-to-roll co-evaporation process for deposition of the CIGS layer. Inthis process, a roll of molybdenum-coated polyimide film (referred to asthe web) is unrolled and moved continuously into and through one or moredeposition zones. In the deposition zones, the web is heated totemperatures of up to ˜450° C. and copper, indium, and gallium areevaporated onto it in the presence of selenium vapor. After passing outof the deposition zone(s), the web cools and is wound onto a take-upspool. See, for example, 2003, Jensen et al., “Back Contact CrackingDuring Fabrication of CIGS Solar Cells on Polyimide Substrates,” NCPVand Solar Program Review Meeting 2003, NREL/CD-520-33586, pages 877-881,which is hereby incorporated by reference in its entirety. Likewise,Birkmire et al., 2005, Progress in Photovoltaics: Research andApplications 13, 141-148, hereby incorporated by reference, disclose apolyimide/Mo web structure, specifically,PI/Mo/Cu(InGa)Se₂/CdS/ZnO/ITO/Ni—Al. Deposition of similar structures onstainless foil has also been explored. See, for example, Simpson et al.,2004, “Manufacturing Process Advancements for Flexible CIGS PV onStainless Foil,” DOE Solar Energy Technologies Program Review Meeting,PV Manufacturing Research and Development, PO32, which is herebyincorporated by reference in its entirety.

In some embodiments of the present invention, an absorber material isdeposited onto a polyimide/molybdenum web, such as those developed byGlobal Solar Energy (Tucson, Ariz.), or a metal foil (e.g., the foildisclosed in Simpson et al.). In some embodiments, the absorber materialis any of the absorbers disclosed herein. In a particular embodiment,the absorber is Cu(InGa)Se₂. In some embodiments, the elongated core ismade of a nonconductive material such as undoped plastic. In someembodiments, the elongated core is made of a conductive material such asa conductive metal, a metal-filled epoxy, glass, or resin, or aconductive plastic (e.g., a plastic containing a conducting filler).Next, the semiconductor junction 410 is completed by depositing a windowlayer onto the absorber layer. In the case where the absorber layer isCu(InGa)Se₂, CdS can be used. Finally, an optional i-layer 415 and TCO412 are added to complete the solar cell. Next, the foil is wrappedaround and/or glued to a wire-shaped or tube-shaped elongated core. Theadvantage of such a fabrication method is that material that cannotwithstand the deposition temperature of the absorber layer, windowlayer, i-layer or TCO layer can be used as an inner core for the solarcell. This manufacturing process can be used to manufacture any of thesolar cells 402 disclosed in the present invention, where the conductivecore 402 comprises an inner core and an outer conductive core. The innercore is any conductive or nonconductive material disclosed hereinwhereas the outer conductive core is the web or foil onto which theabsorber layer, window layer, and TCO were deposited prior to rollingthe foil onto the inner core. In some embodiments, the web or foil isglued onto the inner core using appropriate glue.

An aspect of the present invention provides a method of manufacturing asolar cell comprising depositing an absorber layer on a first face of ametallic web or a conducting foil. Next a window layer is deposited onto the absorber layer. Next a transparent conductive oxide layer isdeposited on to the window layer. The metallic web or conducting foil isthen rolled around an elongated core, thereby forming an elongated solarcell 402. In some embodiments, the absorber layer iscopper-indium-gallium-diselenide (Cu(InGa)Se₂) and the window layer iscadmium sulfide. In some embodiments, the metallic web is apolyimide/molybdenum web. In some embodiments, the conducting foil issteel foil or aluminum foil. In some embodiments, the elongated core ismade of a conductive metal, a metal-filled epoxy, a metal-filled glass,a metal-filled resin, or a conductive plastic.

In some embodiments, a transparent conducting oxide conductive film isdeposited on a wire-shaped or tube-shaped elongated core rather thanwrapping a metal web or foil around the elongated core. In suchembodiments, the wire-shaped or tube-shaped elongated core can be, forexample, a plastic rod, a glass rod, a glass tube, or a plastic tube.Such embodiments require some form of conductor in electricalcommunication with the interior face or back contact of thesemiconductor junction. In some embodiments, divots in the wire-shapedor tube-shaped elongated core are filled with a conductive metal inorder to provide such a conductor. The conductor can be inserted in thedivots prior to depositing the transparent conductive oxide orconductive back contact film onto the wire-shaped or tube-shapedelongated core. In some embodiments such a conductor is formed from ametal source that runs lengthwise along the side of the elongated solarcell 402. This metal can be deposited by evaporation, sputtering, screenprinting, inkjet printing, metal pressing, conductive ink or glue usedto attach a metal wire, or other means of metal deposition.

More specific embodiments will now be disclosed. In some embodiments theelongated core is a glass tubing having a divot that runs lengthwise onthe outer surface of the glass tubing, and the manufacturing methodcomprises depositing a conductor in the divot prior to the rolling step.In some embodiments the glass tubing has a second divot that runslengthwise on the surface of the glass tubing. In such embodiments, thefirst divot and the second divot are on approximate or exact oppositecircumferential sides of the glass tubing. In such embodiments,accordingly, the method further comprises depositing a conductor in thesecond divot prior to the rolling or, in embodiments in which rolling isnot used, prior to the deposition of an inner TCO or conductive film,junction, and outer TCO onto the elongated core.

In some embodiments the elongated core is a glass rod having a firstdivot that runs lengthwise on the surface of the glass rod and themethod comprises depositing a conductor in the first divot prior to therolling. In some embodiments the glass rod has a second divot that runslengthwise on the surface of the glass rod and the first divot and thesecond divot are on approximate or exact opposite circumferential sidesof the glass rod. In such embodiments, accordingly, the method furthercomprises depositing a conductor in the second divot prior to therolling or, in embodiments in which rolling is not used, prior to thedeposition of an inner TCO or conductive film, junction, and outer TCOonto the elongated core. Suitable materials for the conductor are any ofthe materials described as a conductor herein including, but not limitedto, aluminum, molybdenum, titanium, steel, nickel, silver, gold, or analloy thereof.

FIG. 13 details a cross-section of a solar cell 402 in accordance withthe present invention. The solar cell 402 can be manufactured usingeither the rolling method or deposition techniques. Components that havereference numerals corresponding to other embodiments of the presentinvention (e.g., 410, 412, and 420) are made of the same materialsdisclosed in such embodiments. In FIG. 13, there is an elongated tubing1306 having a first and second divot running lengthwise along the tubing(perpendicular to the plane of the page) that are on circumferentiallyopposing sides of tubing 1306 as illustrated. In typical embodiments,tubing 1306 is not conductive. For example, tubing 1306 is made ofplastic or glass in some embodiments. Conductive wiring 1302 is placedin the first and second divot as illustrated in FIG. 13. In someembodiments the conductive wiring is made of any of the conductivematerials of the present invention. In some embodiments, conductivewiring 1302 is made out of aluminum, molybdenum, steel, nickel,titanium, silver, gold, or an alloy thereof. In embodiments where 1304is a conducting foil or metallic web, the conductive wiring 1302 isinserted into the divots prior to wrapping the metallic web orconducting foil 1304 around the elongated core 1306. In embodimentswhere 1304 is a transparent conductive oxide or conductive film, theconductive wiring 1302 is inserted into the divots prior to depositingthe transparent conductive oxide or conductive film 1304 onto elongatedcore 1306. As noted, in some embodiments the metallic web or conductingfoil 1304 is wrapped around tubing 1306. In some embodiments, metallicweb or conducting foil 1304 is glued to tubing 1306. In some embodimentslayer 1304 is not a metallic web or conducting foil. For instance, insome embodiments, layer 1304 is a transparent conductive oxide (TCO).Such a layer is advantageous because it allows for thinner absorptionlayers in the semiconductor junction. In embodiments where layer 1304 isa TCO, the TCO, semiconductor junction 410 and outer TCO 412 aredeposited using deposition techniques.

One aspect of the invention provides a solar cell assembly comprising aplurality of elongated solar cells 402 each having the structuredisclosed in FIG. 13. That is, each elongated solar cell 402 in theplurality of elongated solar cells comprises an elongated tubing 1306, ametallic web or a conducting foil (or, alternatively, a layer of TCO)1304 circumferentially disposed on the elongated tubing 1306, asemiconductor junction 410 circumferentially disposed on the metallicweb or the conducting foil (or, alternatively, a layer of TCO) 1304 anda transparent conductive oxide layer 412 disposed on the semiconductorjunction 410. The elongated solar cells 402 in the plurality ofelongated solar cells are geometrically arranged in a parallel or a nearparallel manner thereby forming a planar array having a first face and asecond face. The plurality of elongated solar cells is arranged suchthat one or more elongated solar cells in the plurality of elongatedsolar cells are not in electrically conductive contact with adjacentelongated solar cells. In some embodiments, the elongated solar cellscan be in physical contact with each other if there is an insulativelayer between adjacent elongated solar cells. The solar cell assemblyfurther comprises a plurality of metal counter-electrodes. Eachrespective elongated solar cell 402 in the plurality of elongated solarcells is bound to a first corresponding metal counter-electrode 420 inthe plurality of metal counter-electrodes such that the first metalcounter-electrode lies in a first groove that runs lengthwise on therespective elongated solar cell 402. The apparatus further comprises atransparent electrically insulating substrate that covers all or aportion of said the face of the planar array. A first and secondelongated solar cell in the plurality of elongated solar cells areelectrically connected in series by an electrical contact that connectsthe first electrode of the first elongated solar cell to the firstcorresponding counter-electrode of the second elongated solar cell. Insome embodiments, the elongated tubing 1306 is glass tubing or plastictubing having a one or more grooves filled with a conductor 1302. Insome embodiments, each respective elongated solar cell 402 in theplurality of elongated solar cells is bound to a second correspondingmetal counter-electrode 420 in the plurality of metal counter-electrodessuch that the second metal counter-electrode lies in a second groovethat runs lengthwise on the respective elongated solar cell 402 and suchthat the first groove and the second groove are on opposite orsubstantially opposite circumferential sides of the respective elongatedsolar cell 402. In some embodiments, the plurality of elongated solarcells 402 is configured to receive direct light from the first face andsaid second face of the planar array.

5.7 Static Concentrators

In some embodiments, static concentrators are used to improve theperformance of the solar cell assemblies of the present invention. Theuse of a static concentrator in one exemplary embodiment is illustratedin FIG. 11, where static concentrator 1102, with aperture AB, is used toincrease the efficiency of bifacial solar cell assembly CD, where solarcell assembly CD is any of 400 (FIG. 4), 600 (FIG. 6), 700 (FIG. 7), 800(FIG. 8), 900 (FIG. 9), or 1000 (FIG. 10). Static concentrator 1102 canbe formed from any static concentrator materials known in the art suchas, for example, a simple, properly bent or molded aluminum sheet, orreflector film on polyurethane. Concentrator 1102 is an example of a lowconcentration ratio, nonimaging, compound parabolic concentrator(CPC)-type collector. Any (CPC)-type collector can be used with thesolar cell assemblies of the present invention. For more information on(CPC)-type collectors, see Pereira and Gordon, 1989, Journal of SolarEnergy Engineering, 111, pp. 111-116, which is hereby incorporated byreference in its entirety.

Additional static concentrators that can be used with the presentinvention are disclosed in Uematsu et al., 1999, Proceedings of the11^(th) International Photovoltaic Science and Engineering Conference,Sapporo, Japan, pp. 957-958; Uematsu et al., 1998, Proceedings of theSecond World Conference on Photovoltaic Solar Energy Conversion, Vienna,Austria, pp. 1570-1573; Warabisako et al., 1998, Proceedings of theSecond World Conference on Photovoltaic Solar Energy Conversion, Vienna,Austria, pp. 1226-1231; Eames et al., 1998, Proceedings of the SecondWorld Conference on Photovoltaic Solar Energy Conversion, ViennaAustria, pp. 2206-2209; Bowden et al., 1993, Proceedings of the 23^(rd)IEEE Photovoltaic Specialists Conference, pp. 1068-1072; and Parada etal., 1991, Proceedings of the 10^(th) EC Photovoltaic Solar EnergyConference, pp. 975-978, each of which is hereby incorporated byreference in its entirety.

In some embodiments, a static concentrator as illustrated in FIG. 12 isused. The bifacial solar cells illustrated in FIG. 12 can be any of thebifacial solar cell assemblies of the present invention, including butnot limited to assembly 400 (FIG. 4), 600 (FIG. 6), 700 (FIG. 7), 800(FIG. 8), 900 (FIG. 9), or 1000 (FIG. 10). The static concentrator usestwo sheets of cover glass on the front and rear of the module withsubmillimeter V-grooves that are designed to capture and reflectincident light as illustrated in the Figure. More details of suchconcentrators is found in Uematsu et al., 2001, Solar Energy Materials &Solar Cell 67, 425-434 and Uematsu et al., 2001, Solar Energy Materials& Solar Cell 67, 441-448, each of which is hereby incorporated byreference in its entirety. Additional static concentrators that can beused with the present invention are discussed in Handbook ofPhotovoltaic Science and Engineering, 2003, Luque and Hegedus (eds.),Wiley & Sons, West Sussex, England, Chapter 12, which is herebyincorporated by reference in its entirety.

5.8 Internal Reflector Embodiments

FIG. 14 illustrates a solar cell assembly 1400 in accordance with thepresent invention. Specifically, FIG. 14 is a perspective view ofrod-shaped (elongated) solar cells 402 electrically arranged in seriesin solar cell assembly 1400 where counter-electrodes 420 are inelectrical communication with internal reflectors 1404. As illustratedin FIG. 14, solar cell assembly 1400 comprises a plurality of elongatedsolar cells 402. There is no limit to the number of solar cells 402 inthis plurality (e.g., 10 or more, 100 or more, 1000 or more, 10,000 ormore, between 5,000 and one million solar cells 402, etc.). As in theembodiment of the invention illustrated in FIG. 4 and described above,each elongated solar cell 402 comprises a conductive core 404 with asemiconductor junction 410 circumferentially disposed on the conductivecore. A transparent conductive oxide layer (TCO) 412 circumferentiallydisposed on the semiconductor junction 410 completes the circuit. Inpreferred embodiments, there is an intrinsic layer 415 circumferentiallydisposed between semiconductor junction 410 and TCO 412. In someembodiments, this intrinsic layer is formed by an undoped transparentoxide such as zinc oxide, or any transparent metal oxide that is highlyinsulating. In FIG. 14, semiconductor junction 410, optional intrinsiclayer 415, and TCO 412 are schematically drawn as a single layer so thatother aspects of the architecture of solar cell assembly 1400 can beemphasized. As can be seen in FIG. 14, each elongated solar cell 402 hasa length that is great compared to the diameter of its cross-section.Typically, each solar cell 402 has a rod-like shape (e.g., has a wireshape). In some embodiments, counter-electrodes 420 are made out ofaluminum or copper.

In typical embodiments there is a first groove 677-1 and a second groove677-2 that each runs lengthwise on opposing sides of solar cell 402. Insome embodiments, the counter-electrodes 420 are fitted into grooves 677in the manner illustrated in FIG. 14. Typically, such counter-electrodes420 are glued into grooves 677 using a conductive ink or conductiveglue. For example, CuPro-Cote (available from Lessemf.com, Albany,N.Y.), which is a sprayable metallic coating system using anon-oxidizing copper as a conductor, can be used. In preferredembodiments, the counter-electrodes 420 are fitted in to grooves 677 andthen a bead of conductive ink or conductive glue is applied. As inprevious embodiments, the present invention encompasses grooves 677 thathave a broad range of depths and shape characteristics and is by nomeans limited to the shape of the grooves 677 illustrated in FIG. 14. Ingeneral, any type of groove 677 that runs along the long axis of a firstsolar cell 402 and that can accommodate all or part of counter-electrode420 is within the scope of the present invention.

Internal reflectors 1404 run lengthwise along corresponding solar cells402 as shown, for example, in FIG. 14. In some embodiments, internalreflectors 1404 have a hollow core. As in the case of elongatedconductive core 404, a hollow core is advantageous in many instancesbecause it reduces the amount of material needed to make such devices,thereby lowering costs. In some embodiments, internal reflector 1404 isa plastic casing with a layer of highly reflective material (e.g.,polished aluminum, aluminum alloy, silver, nickel, steel, etc.)deposited on the plastic casing. In some embodiments internal reflector1404 is a single piece made out of polished aluminum, aluminum alloy,silver, nickel, steel, etc. In some embodiments, internal reflector is ametal or plastic casing onto which is layered a metal foil tape.Exemplary metal foil tapes include, but are not limited to, 3M aluminumfoil tape 425, 3M aluminum foil tape 427, 3M aluminum foil tape 431, and3M aluminum foil tape 439 (3M, St. Paul, Minn.). Internal reflector 1404can adopt a broad range of designs, only one of which is illustrated inFIG. 14. Central to the design of reflectors 1404 found in a preferredembodiment of the present invention is the desire to reflect directlight that enters into both sides of solar cell assembly 1400 (i.e.,side 1420 and side 1430). In general, reflector 1404 is designed tooptimize reflection of light into adjacent elongated solar cells 402.Direct light that enters one side of solar cell assembly 1400 (e.g.,side 1420, above the plane of the solar cell assembly drawn in FIG. 14)is directly from the sun whereas light that enters the other side of thesolar cell (e.g., side 1430, below the plane of the solar cell assemblydrawn in FIG. 14) will have been reflected off of a Lambertian ordiffuse reflector surface. Thus, because each side of the solar cellassembly faces a different light environment, the shape of reflector1404 on side 1420 may be different than on side 1430.

A counter-electrode collar 1402 is found toward the end of eachelongated solar cell 402. Each counter-electrode collar 1402 is made ofa thin strip of conductive material that is wrapped around transparentconductive oxide 412 toward the ends of elongated solar cell 402. Insome embodiments, each counter-electrode collar 1402 is made of aconductive ribbon of metal (e.g., copper, aluminum, gold, silver,molybdenum, or an alloy thereof) or a conductive ink. As will beexplained in conjunction with subsequent drawings, counter-electrodecollar 1402 is used to electrically connect elongated solar cells 402.

In embodiments not illustrated in FIGS. 14-17, elongated solar cells 402are swaged at their ends such that the diameter at the ends is less thanthe diameter towards the center of such cells. Conductive collar 1402 isthen placed on the swaged ends. The purpose of the swaged ends is tofacilitate placement of conductive collars 1402 on the ends of elongatedsolar cells 402 and to allow for the placement of elongated solar cells402 closer together.

As illustrated in FIG. 14, solar cells in the plurality of elongatedsolar cells 402 are geometrically arranged in a parallel or nearparallel manner. However, unlike the embodiments illustrated in FIG. 6,elongated solar cells 402 are not arranged as a plurality of solar cellpairs. In some embodiments, each internal reflector 1404 connects to twoelongated solar cells 402, for example, in the manner illustrated inFIGS. 14 through 17. Because of this, the elongated solar cells 402 areeffectively joined into a single composite device. The way in whichinternal reflectors 1404 interface with elongated solar cells 402, inaccordance with some embodiments of the present invention, is seen moreclearly in FIG. 17, which illustrates a cross-sectional view of a solarcell assembly drawn with respect to line 17-17′ of FIG. 14. In FIG. 17,each internal reflector 1404 is connected to the two adjacent elongatedsolar cells 402 such that the internal reflector 1404 contacts theelongated counter-electrode 420 strips of the two elongated solar cells402 in the manner shown. For example, a first edge of internal reflector1404 connects the reflector to a first counter-electrode 420 of a firstelongated solar cell 402 and a second edge of internal reflector 1404connects the reflector to a second counter-electrode 420 of a secondelongated solar cell 402. In practice, the internal reflector 1404 canbe sealed to such counter-electrode 420 strips using any suitableelectrically conductive glue or ink, such as any of the electricallyconductive glues disclosed in preceding sections. There is no limitationon the cross-sectional shape of reflector 1404. In general, thecross-sectional shape of reflector 1404 is optimized to reflect as muchlight as possible onto the elongated solar cells 402. Although elongatedsolar cells 402 are drawn as circular in FIG. 17, they can have anycross-sectional shape that does not have an edge. For example, in someembodiments, the cross-sectional shape of cells 402 is oval in naturesuch that the long axis of such ovals is parallel or nearly parallel toline 1750-1750′ of FIG. 17. Such an arrangement is advantageous in somesolar cell assemblies because it requires fewer elongated solar cells402 per arbitrary unit of solar panel real estate in the assembly. Sinceelongated solar cells 402 have an associated cost, fewer numbers of suchelongated solar cells 402 covering the same amount of area is more costeffective.

In some embodiments, not shown, internal reflectors 1404 areelectrically isolated from counter-electrodes 420. Thus, in suchembodiments, internal reflectors 1404 do not electrically connect thecounter-electrodes 420 of adjacent elongated solar cells 402. In suchembodiments, internal reflectors 1404 are isolated from elongated solarcells 402 using a transparent insulator such as Sylgard (Dupont, USA)and/or ethyl vinyl acetate, and/or spray Teflon.

An arrangement of internal reflectors 1404 and elongated solar cells402, such as that illustrated in FIGS. 14 and 17, is advantageous forseveral reasons. First, elongated solar cells 402 have appreciablecosts. Thus, an architecture, such as that illustrated in FIGS. 14 and17, that reduces the number of elongated solar cells 402 will reducecosts. In typical embodiments, internal reflectors 1404 cost less tomake than elongated solar cells 402. Furthermore, internal reflectors1404 reflect light onto existing elongated solar cells. This additionalreflective light makes such elongated solar cells 402 more efficientthan comparable elongated solar cells 402 that lack internal reflectors1404. Thus, the reduced number of elongated solar cells 402 perarbitrary unit of solar cell apparatus real estate that is realized by asolar cell architecture, such as that disclosed in FIG. 14, iscompensated for by the increased electrical output of the elongatedsolar cells 402 owing to the additional reflected light provided byinternal reflectors 1404. This means that, although the density of solarcells 402 in solar cell assembly 1400 relative to comparablearchitectures described in conjunction with previous figures above isreduced, such reduced elongated solar cell 402 density is compensatedfor by the fact that the electrical output of the elongated solar cells402 is greater in solar cell assembly 1400 because of the lightreflected by internal reflectors 1404.

In some embodiments, another reason that the arrangement of internalreflectors 1404 and elongated solar cells 402, such as that illustratedin FIGS. 14 and 17, is advantageous is that the reflective surface oninternal reflectors 1404 (or the internal reflectors 1404 themselves inthe case where such reflectors are made from a single piece ofconductive metal) can conduct electricity. Thus, they can be used tolower the conductive burden on counter-electrodes 1402. With theirreduced conductive burden, counter-electrodes 1402 can be made smallerthan counter-electrodes 1420 found in comparable solar cellarchitectures such as those illustrated in preceding sections. This isadvantageous because the materials used to make counter-electrodes 1402(e.g., copper, etc.) are costly. Any reduction in the size of suchcounter-electrodes significantly lowers the cost of manufacturing solarcell assembly 1400. In some embodiments, reflectors 1404 are used tolower the conductive burden on counter-electrodes 420 by electricallyconnecting to adjacent counter-electrodes 420 in the manner illustratedin FIG. 17. In preferred embodiments, elongated solar cells 402 areconnected in series, not in parallel. Therefore, in embodiments wherereflectors 1404 are electrically connected to adjacentcounter-electrodes 420, it is preferable to preserve the serialelectrical arrangement of elongated solar cells 402. This can beaccomplished by electrically isolating the portion of an internalreflector 1404 that contacts a first counter-electrode 420 from theportion of the internal reflector 1404 that contacts a secondcounter-electrode 420. There are many ways in which this electricalisolation can be accomplished. For example, the center of internalreflector 1404 can comprise an insulative material that insulates theportion of internal reflector 1404 that contacts one counter-electrode420 from the portion of internal reflector 1404 that contacts the othercounter-electrode 420. In another example, the internal reflector 1404comprises an insulative core with a layer of reflective materialdeposited on the insulative core. The layer of reflective materialcontacts adjacent counter-electrodes 420. However, to preserve thein-series electrical connection of the elongated solar cells 402, thereare breaks in the reflective, conductive layer. In one example,referring to FIG. 17, there is a first break in the reflective materialat or near the apex of internal reflector 1404 that faces side 1420 andthere is a second break in the reflective material at or near the apexof internal reflector 1404 that faces side 1430.

In some embodiments elongated solar cells 402 are on the order of ameter long and there is less than a two percent resistive loss along thelength of counter-electrode strips 420. In exemplary embodiments whereinternal reflectors 1404 are not used or are electrically isolated fromcounter-electrodes 420, to ensure that the resistive loss is less thantwo percent, and assuming that there are two counter-electrode strips420 per elongated solar cell 402, that such counter-electrode strips 420adopt a wire shape, and that such counter-electrode strips 420 are madeof pure copper that have a conductivity of 1.7 μOhm-cm, the diameter ofelectrode strips 420 is on the order of 1 millimeter. In embodimentswhere internal reflector 1404 is used and is electrically connected tocounter-electrodes 420, counter-electrode strips 420 can be of smallerdiameter and still have less than a two percent resistive loss along thelength of the electrodes. In some embodiments, solar cells 402 have alength that is between 0.5 meters and 2 meters. In some embodiments,counter-electrodes 420 have a diameter that falls within the rangebetween 0.5 millimeters and 1.5 millimeters.

Even though solar cells 402 form a single composite device in solar cellassembly 1400, in preferred embodiments individual elongated solar cells402 are electrically arranged in series rather than in parallel. In someembodiments, an in-series rather than in-parallel architecture isaccomplished by implementing specific design features. Such features canbe seen with greater clarity in FIG. 15, which is an enlarged portion ofthe solar cell assembly 1400 of FIG. 14. In FIG. 15, each electricalcontact 690 electrically connects the conductive core 404 of oneelongated solar cell 402 to a counter-electrode collar 1402 of anadjacent elongated solar cell 402.

In some embodiments the material used to make electrical contact 690 forsolar cell assembly 1400 is a conductive tape. In other preferredembodiments, electrical contact 690 is made out of an electricallyconducting material such as copper or aluminum. Consider the case inwhich electrical contact 690 is made out of a conductive tape. Such tapetypically has an adhesive conductive surface. In FIG. 15, the adhesiveconductive surface is face down into the page such that it contacts theelongated conductive core 404 of the elongated solar cell 402 on theright and such that it contacts the counter-electrode collar 1402 of theelongated solar cell 402 on the left. Exemplary conductive tapesinclude, but are not limited to, ZTAPE available from iEC (CommerceCity, Colo.). However, in general, any conductive tape comprising anadhesive with a backing onto which is deposited a conductive material(e.g., silver, tin, nickel, copper, graphite, or aluminum) will suffice.

Care is taken so that electrical contact 690 does not contact internalmirror 1404 thereby causing a short. In some embodiments, elongatedconductive core 404 is any of the dual layer cores described in Section5.4. In such embodiments, the terminal ends of elongated solar cells 402can be stripped down to either the inner core or the outer core. Forexample, consider the case in which elongated solar cell 402 is made outof an inner core made of aluminum and an outer core made of molybdenum.In such a case, the end of elongated solar cell 402 can be stripped downto the molybdenum outer core and the contact 690 electrically connectedwith this outer core. Alternatively, the end of elongated solar cell 402can be stripped down to the aluminum inner core and the contact 690electrically connected with this inner core. As illustrated in FIG. 15,contacts 690 that are in electrical contact with the conductive core 404of a given elongated solar cell 402 do not contact the counter-electrodecollar 1402 of the given elongated solar cell 402 because such a contactwould cause an electrical short.

In some embodiments, contact 690 and the counter-electrode collar 1402that contact 690 is in electrical contact with are formed out a singlepiece that is patterned such that collar 1402 wraps around theappropriate elongated solar cell 402. It should be emphasized thatalthough illustrated as such in FIGS. 14-17, there is no requirementthat counter-electrode collar 1402 wrap all the way around elongatedconductive core 404. All that is required is that collar 1402 form anelectrical contact with counter-electrodes 420. In some embodiments,however, there is a contact 690 on both the top surface, as illustratedin FIG. 15, and the bottom surface (such contacts 690 are notillustrated in FIG. 15). In such embodiments, it is necessary for collar1402 to wrap around the entire circumference of elongated conductivecore 404. In some embodiments where there is a contact 690 on both thetop surface and the bottom surface, elongated solar cells 402 arestaggered.

Referring to FIG. 16, the serial architecture of solar cell assembly1400 is made more apparent. FIG. 16 depicts the same solar cell assembly1400 shown in FIG. 14, the exception being that electrical contacts 690are shown in FIG. 16 whereas they are not shown in FIG. 14. As can beseen in FIG. 16, solar cells 402 are electrically connected to eachother in a series manner by electrical contacts 690. Another feature ofsolar cell assembly 1400 that is made apparent in FIG. 16 is that thereare electrical contacts 690 on each end of elongated solar cells 402.Such an electrical arrangement is advantageous because it reduces burdenon counter-electrodes 420. Current only has to travel half the distancein any given elongated solar cell 402 in such an architecture. Thus, thediameter of the wire needed for counter-electrodes 420 in order toensure a less than two percent resistive loss across the length of theelongated solar cells is substantially less than embodiments whereelectrical contacts 690 are found only at a single given end ofelongated solar cells 402.

Referring once again to FIG. 17, a transparent electrically insulatingsubstrate 406 covers all or a portion of face 1430 of the planar arrayof solar cells. In some embodiments, solar cells 402 contact substrate406. In some embodiments, solar cells 402 do not contact substrate 406.In embodiments in which elongated solar cells 402 do not contactsubstrate 406, a sealant such as ethyl vinyl acetate (EVA) is used toseal substrate 406 onto solar cells 402. In fact, a sealant such as EVAcan also be used to seal assembly 400 even in embodiments in whichelongated solar cells 402 do contact substrate 406. As illustrated inFIG. 17, there is a layer 1702 of sealant that seals the assembly tosubstrate 406. In some embodiments, layer 1702 is EVA that has beenapplied in liquid form in order to reach into crevices thereby forminglayer 1702. Then, electrically resistant transparent substrate 406 andcovering 422 are positioned on the EVA filler such that they touch ornearly touch the tops of the elongated solar cells 402 and reflectors1404. Thus, in typical embodiments, layer 1702 of EVA does notsignificantly clear the features of the elongated solar cells 402 andreflectors 1404 and resistive transparent substrate 406 and covering 422nearly touch each other. In such embodiments, the thickness of layer1402 is simply the thickness of elongated solar cells and/or reflectors1404.

In some embodiments, solar cell assembly 1400 further comprises atransparent insulating covering 422 disposed on face 1420 of the planararray of solar cells 402, thereby encasing the plurality of elongatedsolar cells 402 between the transparent insulating covering 422 and thetransparent electrically insulating substrate 406. In such embodiments,transparent insulating covering 422 and the transparent insulatingsubstrate 406 are bonded together by a sealant such as ethyl vinylacetate.

In some embodiments, the semiconductor junction 410 of solar cells 402in assembly 1400 comprise an inner coaxial layer and an outer coaxiallayer, where the outer coaxial layer comprises a first conductivity typeand the inner coaxial layer comprises a second, opposite, conductivitytype. In some embodiments, the inner coaxial layer comprisescopper-indium-gallium-diselenide (CIGS) and the outer coaxial layercomprises CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments,conductive core 404 and/or electrical contacts 677 and/orcounter-electrodes 420 are made of aluminum, molybdenum, steel, nickel,silver, gold, or an alloy thereof. In some embodiments, transparentconductive oxide layer 412 is made of tin oxide SnO_(x), with or withoutfluorine doping, indium-tin oxide (ITO), zinc oxide (ZnO),indium-zinc-oxide or a combination thereof. In some embodiments,transparent insulating substrate 406 and transparent insulating covering422 comprise glass or Tedlar.

In some embodiments elongated solar cells 402 are cooled by pumping airor helium through the hollow channels at the center of such solar cells402. In still other embodiments, solar cells 402 and/orcounter-electrode collar 1402 do not have a hollow core as depicted inFIGS. 14 through 17.

5.9 Additional Internal Reflector Embodiments

FIG. 18 illustrates an elongated solar cell 402 in accordance with thepresent invention. As depicted in FIG. 18, the coating on elongatedsolar cell 402 is not uniform. The ends of the elongated solar cell 402are stripped and conductive layer 404/1304 is exposed. As in previousembodiments, the conductive layer (elongated conductive core) 404/1304serves as the first electrode in the assembly and transparent conductiveoxide (TCO) 412 on the exterior surface of each elongated solar cell 402serves as the counter-electrode. In some embodiments in accordance withthe present invention as illustrated in FIG. 18, however, protrudingcounter-electrodes 420 and electrodes 440, which are attached to theelongated solar cell 402, provide convenient electrical connection.

In typical embodiments as shown in FIG. 18, there is a first groove677-1 and a second groove 677-2 that each runs lengthwise on opposingsides of elongated solar cell 402. In some embodiments,counter-electrodes 420 are fitted into grooves 677 in the mannerillustrated in FIG. 18. Typically, such counter-electrodes 420 are gluedinto grooves 677 using a conductive ink or conductive glue. For example,CuPro-Cote (available from Lessemf.com, Albany, N.Y.), which is asprayable metallic coating system using a non-oxidizing copper as aconductor, can be used. In preferred embodiments, counter-electrodes 420are fitted in to grooves 677 and then a bead of conductive ink orconductive glue is applied. As in previous embodiments, the presentinvention encompasses grooves 677 that have a broad range of depths andshape characteristics and is by no means limited to the shape of thegrooves 677 illustrated in FIG. 18. In general, any type of groove 677that runs along the long axis of a first solar cell 402 and that canaccommodate all or part of counter-electrode 420 is within the scope ofthe present invention. Counter-electrodes 420 conduct current from thecombined layer 410/(415)/412. At the regions at both ends of elongatedsolar cell 402, counter-electrodes 420 are sheathed as shown in FIG. 18so that they are electrically isolated from conductive layer 404/1304.The ends of protruding counter-electrodes 420, however, are unsheathedso they can form electric contact with additional devices.

In the embodiments as depicted in FIG. 18, a second set of electrodes440 are attached to the exposed conductive layer 404/1304. The secondset of electrodes 440 conduct current from the exposed conductive layer404/1304. As illustrated in FIG. 18, an embodiment in accordance withthe present invention has two electrodes 440 attached at two opposingends of each elongated solar cell 402. Typically, electrodes 440 areglued onto layer 404/1304 using a conductive ink or conductive glue. Forexample, CuPro-Cote (available from Lessemf.com, Albany, N.Y.), which isa sprayable metallic coating system using a non-oxidizing copper as aconductor, can be used. In preferred embodiments, electrodes 440 areglued to layer 404/1304 and then a bead of conductive ink or conductiveglue is applied. Care is taken so that electrodes 440 are not inelectrical contact with layer 410/(415)/412. Additionally, electrodes440 in the present invention have a broad range of lengths and widthsand shape characteristics and are by no means limited to the shape of440 illustrated in FIG. 18. In the embodiments as shown in FIG. 18, thetwo electrodes 440 on opposite ends of the elongated solar cell 402 arenot on the same side of the solar cell tube. The first electrode 440 ison the bottom side of the elongated solar cell 402 while the secondelectrode 440 is on the top side of the elongated solar cell 402. Suchan arrangement facilitates the connection of the solar cells in a serialmanner. In some embodiments in accordance with the present invention,the two electrodes 440 can be on the same side of the elongated solarcell 402.

In some embodiments, each electrode 440 is made of a thin strip ofconductive material that is attached to conductive layer 404/1304 (FIG.18). In some embodiments, each electrode 440 is made of a conductiveribbon of metal (e.g., copper, aluminum, gold, silver, molybdenum, or analloy thereof) or a conductive ink. As will be explained in conjunctionwith subsequent drawings, counter-electrode 420 and electrodes 440 areused to electrically connect elongated solar cells 402, preferably inseries.

FIG. 19 illustrates a solar cell assembly 1900 in accordance with thepresent invention. The elongated solar cells 402 and reflector 1404 areassembled into an alternating array as shown. Elongated solar cells 402in solar cell assembly 1900 have been equipped with counter-electrodes420 and electrodes 440. As illustrated in FIG. 19, solar cell assembly1900 comprises a plurality of elongated solar cells 402. There is nolimit to the number of solar cells 402 in this plurality (e.g., 10 ormore, 100 or more, 1000 or more, 10,000 or more, between 5,000 and onemillion solar cells 402, etc.). Accordingly, solar cell assembly 1900also comprises a plurality of reflectors 1404. There is no limit to thenumber of reflectors 1404 in this plurality (e.g., 10 or more, 100 ormore, 1000 or more, 10,000 or more, between 5,000 and one millionreflector 1404, etc.).

Within solar cell assembly 1900, internal reflectors 1404 run lengthwisealong corresponding elongated solar cells 402 as shown, for example, inFIG. 19. In some embodiments, internal reflectors 1404 have a hollowcore. As in the case of elongated conductive core 404, a hollow core isadvantageous in many instances because it reduces the amount of materialneeded to make such devices, thereby lowering costs. In someembodiments, internal reflector 1404 is a plastic casing with a layer ofhighly reflective material (e.g., polished aluminum, aluminum alloy,silver, nickel, steel, etc.) deposited on the plastic casing. In someembodiments, internal reflector 1404 is a single piece made out ofpolished aluminum, aluminum alloy, silver, nickel, steel, etc. In someembodiments, internal reflector 1404 is a metal or plastic casing ontowhich is layered a metal foil tape. Exemplary metal foil tapes include,but are not limited to, 3M aluminum foil tape 425, 3M aluminum foil tape427, 3M aluminum foil tape 431, and 3M aluminum foil tape 439 (3M, St.Paul, Minn.). Internal reflector 1404 can adopt a broad range ofdesigns, only one of which is illustrated in FIG. 19. Central to thedesign of reflectors 1404 found in a preferred embodiment of the presentinvention is the desire to reflect direct light that enters into bothsides of solar cell assembly 1900 (i.e., side 1920 and side 1940).

In general, the reflectors 1404 of the present invention are designed tooptimize reflection of light into adjacent elongated solar cells 402.Direct light that enters one side of solar cell assembly 1900 (e.g.,side 1940, above the plane of the solar cell assembly drawn in FIG. 19)is directly from the sun whereas light that enters the other side of thesolar cell (e.g., side 1920, below the plane of the solar cell assemblydrawn in FIG. 19) will have been reflected off of a surface. In someembodiments, this surface is Lambertian, a diffuse or an involutereflector. Thus, because each side of the solar cell assembly faces adifferent light environment, the shape of internal reflector 1404 onside 1920 may be different than on side 1940.

Although the internal reflector 1404 is illustrated in FIG. 19 as havinga symmetrical four-sided cross-sectional shape, the cross-sectionalshape of the internal reflectors 1404 of the present invention are notlimited to such a configuration. In some embodiments, a cross-sectionalshape of an internal reflector 1404 is astroid. In some embodiments, across-sectional shape of an internal reflector 1404 is four-sided and atleast one side of the four-sided cross-sectional shape is linear. Insome embodiments, a cross-sectional shape of an internal reflector 1404is four-sided and at least one side of the four-sided cross-sectionalshape is parabolic. In some embodiments, a cross-sectional shape of aninternal reflector 1404 is four-sided and at least one side of thefour-sided cross-sectional shape is concave. In some embodiments, across-sectional shape of an internal reflector 1404 is four-sided; andat least one side of the four-sided cross-sectional shape is circular orelliptical. In some embodiments, a cross-sectional shape of an internalreflector in the plurality of internal reflectors is four-sided and atleast one side of the four-sided cross-sectional shape defines a diffusesurface on the internal reflector. In some embodiments, across-sectional shape of an internal reflector 1404 is four-sided and atleast one side of the four-sided cross-sectional shape is the involuteof a cross-sectional shape of an elongated solar cell 402. In someembodiments a cross-sectional shape of an internal reflector 1404 is atwo-sided, three-sided, four-sided, five-sided, or six-sided. In someembodiments, a cross-sectional shape of an internal reflector in theplurality of internal reflectors is four-sided and at least one side ofthe four-sided cross-sectional shape is faceted.

Additional features are added to reflectors 1404 to enhance thereflection onto adjacent elongated solar cells 402. The modifiedreflectors 1404 are equipped with a strong reflective property such thatincident light is effectively reflected off the side surfaces 1910 ofthe reflectors 1404. In some embodiments, the reflected light offsurfaces 1910 does not have directional preference. In otherembodiments, the reflector surfaces 1910 are designed such that thereflected light is directed towards the elongated solar cell 402 foroptimal absorbance.

Referring to FIG. 17 as a guide, in some embodiments in accordance withthe present invention, connections between elongated solar cells 402 andinternal reflectors 1404 may be provided by indentation on internalreflectors 1404. Such indentations are not shown in FIG. 17. However,FIG. 17 does illustrate how each internal reflector interfaces with twoadjacent elongated solar cells. In some embodiments, the edges oninternal reflectors 1404 that contact the sides of the elongated solarcells 402 are shaped or molded so that the resulting indentations oninternal reflectors 1404 accommodate the circular cross-sectional shapeof adjacent elongated solar cells 402. In some embodiments, elongatedsolar cells 402 do not have a circular cross-section. Nonetheless,indentations on internal reflectors 1404 may be formed so that theinternal reflectors 1404 accommodate the cross-sectional shape ofelongated solar cells 402.

In some embodiments the above-described molded internal reflector design1404 enhances the electrical connectivity between an internal reflectorand its adjacent elongated solar cells 402. Accordingly, in someembodiments, the internal reflector is coated with an electricallyconducing reflective material, such as aluminum or silver, thatfacilitates the action of counter-electrode 420. In such instances, thelobe of internal reflector 1404 facing or electrically connected to thefirst of the two elongated solar cells 402 must be electrically isolatedfrom the lobe of internal reflector 1404 facing or electricallyconnected to the second of the two elongated solar cells to which thereflector is adjoined. Such electrical isolation can be achieved byinterrupting the layer of conductive material at the top and bottom apexof internal reflector 1404. Such electrical isolation can also beachieved by inserting a central electrically insulating portion withininternal reflector 1404 that electrically isolates the two halves of thereflector from each other. In this way, the advantageous serialconnectivity of the elongated solar cells is maintained. In someembodiments, the molded internal reflector design can be used to removea need for discrete counter-electrode wires 420 on elongated solar cells402. Thus, in some embodiments of the present invention, grooves 677such as found in FIG. 18 are not necessary and counter-electrode strips(or wires) 420 are not present. Rather, each electrically isolated halfof internal reflector 1404 serves as a counter-electrode.

In some embodiments, the connection between an internal reflector 1404and an adjacent elongated solar cell is provided by an additionaladaptor piece. Such an adapter piece has surface features that arecomplementary to both the shapes of internal reflectors 1404 as well aselongated solar cells 402 in order to provide a tight fit between suchcomponents. In some embodiments, such adaptor pieces are fixed oninternal reflectors 1404. In other embodiments, the adaptor pieces arefixed on elongated solar cells 402. In additional embodiments, theconnection between elongated solar cells 402 and reflectors 1404 may bestrengthened by electrically conducting glue or tapes. DiffuseReflection.

In some embodiments in accordance with the present invention, the sidesurface 1910 of reflector 1404 is a diffuse reflecting surface (e.g.1910 in FIG. 19). The concept of diffuse reflection can be betterappreciated with a first understanding of the concept of specularreflection. Specular reflection is defined as the reflection off smoothsurfaces such as mirrors or a calm body of water (e.g. 2402 in FIG.24A). On a specular surface, light is reflected off mainly in thedirection of the reflected ray and is attenuated by an amount dependentupon the physical properties of the surface. Since the light reflectedfrom the surface is mainly in the direction of the reflected ray, theposition of the observer (e.g. the position of the elongated solar cells402) determines the perceived illumination of the surface. Specularreflection models the light reflecting properties of shiny ormirror-like surfaces. In contrast to specular reflection, reflection offrough surfaces such as clothing, paper, and the asphalt roadway leads toa different type of reflection known as diffuse reflection (FIG. 24B).Light incident on a diffuse reflection surface is reflected equally inall directions and is attenuated by an amount dependent upon thephysical properties of the surface. Since light is reflected equally inall directions the perceived illumination of the surface is notdependent on the position of the observer or receiver of the reflectedlight (e.g. the position of the elongated solar cell 402). Diffusereflection models the light reflecting properties of matt surfaces.

Diffuse reflection surfaces reflect off light with no directionaldependence for the viewer. Whether the surface is microscopically roughor smooth has a tremendous impact upon the subsequent reflection of abeam of light. Input light from a single directional source is reflectedoff in all directions on a diffuse reflecting surface (e.g. 2404 in FIG.24B). Diffuse reflection originates from a combination of internalscattering of light, e.g. the light is absorbed and then re-emitted, andexternal scattering from the rough surface of the object. LambertianReflection.

In some embodiments in accordance with the present invention, surface1910 of reflector 1404 is a Lambertian reflecting surface (e.g. 2406 inFIG. 24C). A Lambertian source is defined as an optical source thatobeys Lambert's cosine law, i.e., that has an intensity directlyproportional to the cosine of the angle from which it is viewed (FIG.24C). Accordingly, a Lambertian surface is defined as a surface thatprovides uniform diffusion of incident radiation such that its radiance(or luminance) is the same in all directions from which it can bemeasured (e.g., radiance is independent of viewing angle) with thecaveat that the total area of the radiating surface is larger than thearea being measured.

On a perfectly diffusing surface, the intensity of the light emanatingin a given direction from any small surface component is proportional tothe cosine of the angle of the normal to the surface. The brightness(luminance, radiance) of a Lambertian surface is constant regardless ofthe angle from which it is viewed.

The incident light {right arrow over (l)} strikes a Lamertian surface(FIG. 24C) and reflects in different directions. When the intensity of{right arrow over (l)} is defined as I_(in), the intensity (e.g.I_(out)) of a reflected light {right arrow over (v)} can be defined asfollowing in accordance to Lambert's cosine law:

${I_{out}\left( \overset{\rightharpoonup}{v} \right)} = {{I_{in}\left( \overset{\rightharpoonup}{l} \right)}{\varphi\left( {\overset{\rightharpoonup}{v},\overset{\rightharpoonup}{l}} \right)}\frac{\cos\;\theta_{in}}{\cos\;\theta_{out}}}$

where φ({right arrow over (v)},{right arrow over (l)})=k_(d) cos θ_(out)and k_(d) is related to the surface property. The incident angle isdefined as θ_(in), and the reflected angle is defined as θ_(out). Usingthe vector dot product formula, the intensity of the reflected light canalso be written as:I _(out)({right arrow over (v)})=k _(d) I _(in)({right arrow over(l)}){right arrow over (l)}•{right arrow over (n)},

where {right arrow over (n)} denotes a vector that is normal to theLambertian surface.

Such a Lambertian surface does not lose any incident light radiation,but re-emits it in all the available solid angles with a 2π radians, onthe illuminated side of the surface. Moreover, a Lambertian surfaceemits light so that the surface appears equally bright from anydirection. That is, equal projected areas radiate equal amounts ofluminous flux. Though this is an ideal, many real surfaces approach it.For example, a Lambertian surface can be created with a layer of diffusewhite paint. The reflectance of such a typical Lambertian surface may be93%. In some embodiments, the reflectance of a Lambertian surface may behigher than 93%. In some embodiments, the reflectance of a Lambertiansurface may be lower than 93%. Lambertian surfaces have been widely usedin LED design to provide optimized illumination, for example in U.S.Pat. No. 6,257,737 to Marshall, et al.; U.S. Pat. No. 6,661,521 toStern; and U.S. Pat. No. 6,603,243 to Parkyn, et al., which are herebyincorporated by reference in their entireties.

Advantageously, Lambertian surfaces 1910 on reflector 1404 effectivelyreflect light in all directions. The reflected light is then directedtowards the elongated solar cell 402 to enhance solar cell performance.Reflection on Involute Surfaces.

In some embodiments in accordance with the present invention, surface1910 of the reflector 1404 is an involute surface of the elongated solarcell tube 402 (e.g. 2504 in FIG. 25A). In some embodiments, theelongated solar cell tube 402 is circular or near circular. Thereflector surface 1910 is preferably the involute of a circle (e.g. 2504in FIG. 25A). The involute of circle 2502 is defined as the path tracedout by a point on a straight line that rolls around a circle. Forexample, the involute of a circle can be drawn in the following steps.First, attach a string to a point on a curve. Second, extend the stringso that it is tangent to the curve at the point of attachment. Third,wind the string up, keeping it always taut. The locus of points tracedout by the end of the string (e.g. 2504 in FIG. 25) is called theinvolute of the original circle 2502. The original circle 2502 is calledthe evolute of its involute curve 2504.

Although in general a curve has a unique evolute, it has infinitely manyinvolutes corresponding to different choices of initial point. Aninvolute can also be thought of as any curve orthogonal to all thetangents to a given curve. For a circle of radius r, at any time t, itsequation can be written as:x=r cos ty=r sin t*

Correspondingly, the parametric equation of the involute of the circleis:x _(i) =r(cos t+t sin t)y _(i) =r(sin t−t cos t)*

Evolute and involute are reciprocal functions. The evolute of aninvolute of a circle is a circle.

Involute surfaces have been implemented in numerous patent designs tooptimize light reflections. For example, a flash lamp reflector (U.S.Pat. No. 4,641,315 to Draggoo, hereby incorporated by reference in itsentirety) and concave light reflector devices (U.S. Pat. No. 4,641,315to Rose, hereby incorporated by reference in its entirety), which arehereby incorporated by reference in their entireties, both utilizeinvolute surfaces to enhance light reflection efficiency.

In FIG. 25B, an internal reflector 1404 is connected to two elongatedsolar cells 402. Details of both reflector 1404 and solar cell 402 areomitted to highlight the intrinsic relationship between the shapes ofthe elongated solar cell 402 and the shape of the side surface 1910 ofthe internal reflector 1404. Side surfaces 1910 are constructed suchthat they are the involute of the circular elongated solar cell 402.

Advantageously, the involute-evolute design imposes optimal interactionsbetween the side surfaces 1910 of reflectors 1404 and the adjacentelongated solar cell 402. When the side surface 1910 of the reflector1404 is an involute surface corresponding to the elongated solar cell402 that is adjacent or attached to the reflector 1404, light reflectseffectively off the involute surface in a direction that is optimizedtowards the elongated solar cell 402.

Conductive Paint.

In some embodiments in accordance with the present invention, a layer ofconductive paint is deposited on surfaces 1910 of reflectors 1404. Inother embodiments, a layer of some other form of conductive reflectivematerial such as aluminum or silver is deposited on surfaces 1910 ofreflectors 1404. The conductive material on surface 1910 helps toconduct electric current along elongated solar cell 402. By way ofillustration, the methods and designs of only one side-surface 1910 willbe described in detail in the following description. The methods anddesigns, however, apply to all such surfaces.

In embodiments where surfaces 1910 are coated with conductive material,the part of counter-electrodes 420 that run along grooves 677 may beminimized or even omitted. The protruding parts of counter-electrodes420, however, remain in the design even when counter-electrodes 420 areomitted. In embodiments where counter-electrodes 420 do run the entirelength of elongated solar cells 402, the free electrons are conducted bythe conductive surfaces 1910. Furthermore, in embodiments wherecounter-electrodes 420 do not run the entire length of elongated solarcells 402, grooves 677 can be omitted.

As depicted in FIGS. 19 through 23, reflectors 1404 have across-sectional shape that is astroid shape with four equal reflectingsides. However, it will be appreciated that, although the illustratedsymmetrical astroid design is advantageous, the present invention is notrestricted or limited to such cross-sectional shapes.

In embodiments not illustrated in FIGS. 19 through 23, elongated solarcells 402 are swaged at their ends such that the diameter at the ends isless than the diameter towards the center of such cells. Electrodes 440are placed on these swaged ends.

Solar Cell Assembly.

As illustrated in FIG. 19, solar cells in the plurality of elongatedsolar cells 402 are geometrically arranged in a parallel or nearparallel manner. In some embodiments, each internal reflector 1404connects to two elongated solar cells 402, for example, in the mannerillustrated in FIGS. 19 through 23. Because of this, elongated solarcells 402 are effectively joined into a single composite device. The wayin which internal reflectors 1404 interface with elongated solar cells402, in accordance with some embodiments of the present invention, isseen more clearly in FIG. 17, which illustrates a cross-sectional viewof a solar cell assembly 1400 drawn with respect to line 17-17′ of FIG.14. Although the solar cell assembly 1900 (FIG. 19) differs from thesolar cell assembly 1400 (FIG. 14), a cross-sectional view of someembodiments of solar cell assembly 1900 will be highly similar, if notidentical, to that shown in FIG. 17.

In FIG. 19, electrodes 440 extend the connection from the conductivecore 404/1304. In FIG. 20, electrodes 440 and counter-electrodes 420 areconnected to end capping modules 2002. End capping modules 2002 arecircuit board based devices that provide a platform for serialelectrical connection between electrodes 440 and counter-electrodes 420.End capping modules 2002 contain slots for electrodes 440 andcounter-electrodes 420 and provide connection between electrodes 440 andcounter-electrodes 420 (e.g. as shown in FIG. 20). In FIG. 20, endcapping modules 2002 contact electrodes 440 and counter-electrodes 420of the elongated solar cell 402 simultaneously.

In some embodiments, elongated conductive core 404 is any of the duallayer cores described in Section 5.4. In such embodiments, the terminalends of elongated solar cells 402 can be stripped down to either theinner core or the outer core. For example, consider the case in whichelongated solar cell 402 is constructed out of an inner core made ofaluminum and an outer core made of molybdenum. In such a case, the endof elongated solar cell 402 can be stripped down to the molybdenum outercore and the electrode 440 electrically connected with this outer core.Alternatively, the end of elongated solar cell 402 can be stripped downto the aluminum inner core and the electrodes 440 electrically connectedwith this inner core.

Referring back to FIG. 20, in some embodiments, the end capping module2002 is a circuit board based module that provides electricalcommunication between the electrodes 440 and counter-electrodes 420 atboth ends of the elongated solar cells 402. End capping modules 2002contain slots into which electrodes 420 and 440 fit. Two end cappingmodules are associated with each solar cell assembly. On the end cappingmodules 2002, connection points 2006 correspond to counter-electrodes420, and connection points 2004 correspond to electrodes 440. Electricalcommunication can be established by linking 2004 and 2006 in a serialconnection. Electrical leads within capping module 2002 (not shown) makesuch connections.

Referring to FIG. 21, the end capping modules 2002 are attached to bothends of the solar cell-reflector assembly 1900 to form a completelycapped solar cell assembly 2100. Referring to FIG. 22, the capped solarcell assembly 2100 is placed into an assembly frame 2202. Assembly frame2202 provides a structure for the solar cell assembly 1900. Assemblyframe 2202 can be made of any transparent material, such as plastic orglass. Referring to FIG. 23, the entire assembly is sealed betweensheets of EVA. Then, not shown, the assembly is sandwiched betweenplates of a rigid substrate, such as glass or polyvinyl fluorideproducts (e.g., Tedlar or Tefzel).

5.9 Additional Internal Reflector Embodiments

FIG. 18 illustrates an elongated solar cell 402 in accordance with thepresent invention. As depicted in FIG. 18, the coating on elongatedsolar cell 402 is not uniform. The ends of the elongated solar cell 402are stripped and conductive layer 404/1304 is exposed. As in previousembodiments, the conductive layer (elongated conductive core) 404/1304serves as the first electrode in the assembly and transparent conductiveoxide (TCO) 412 on the exterior surface of each elongated solar cell 402serves as the counter-electrode. In some embodiments in accordance withthe present invention as illustrated in FIG. 18, however, protrudingcounter-electrodes 420 and electrodes 440, which are attached to theelongated solar cell 402, provide convenient electrical connection.

5.10 Embodiments with Lateral Contact Between Counter-Electrodes andConductive Core

FIGS. 26A-C illustrate solar cell assembly 2600 in accordance withanother embodiment of the present invention. Solar cell assembly 2600comprises a plurality of elongated solar cells 402. Each elongated solarcell 402 in the plurality of elongated solar cells has a core 404configured as a first electrode, a semiconductor junction 410circumferentially disposed on core 404 and a transparent conductiveoxide (TCO) layer 412 disposed on the semiconductor junction 410. Core404, semiconductor junction 410 and TCO layer may comprise any of thesame materials, properties and dimensions as described above forcorresponding elements having the same respective item numbers describedabove in Sections 5.1 to 5.6. In some embodiments core 404 comprises anonconductive inner core that is circumferentially coated with anelectrically conductive layer. The plurality of elongated solar cells402 are geometrically arranged in a parallel or a near parallel manner,thereby forming a planar array having a first face (facing side 2633 ofassembly 2600) and a second face (facing side 2666 of assembly 2600).

Each solar cell 402 includes an elongated counter-electrode 420 and anotch 2620. Elongated counter-electrode 420 and notch 2620 eachrespectively run lengthwise on approximately opposing sides of solarcell 402. Notch 2620 may be any disruption, e.g., a notch, scratch,break, void, channel, cavity or other disruption, generally referred toherein as a “notch”, formed in the outer layers 410, 412 to expose theconductive core 404 or a conductive layer surrounding 404 (e.g., layer2610 in FIG. 26B). In FIG. 26A, some but not all notches 2620 arelabeled. Each notch 2620 extends through TCO layer 412 and semiconductorjunction 410 to conductive core 404. Counter-electrode 420 extendsoutward from TCO layer 412 and is dimensioned to fit within notch 2620of adjacent solar cell 402 such that counter-electrode 420 contactsouter edge of conductive core 404 of the adjacent solar cell 402. Notch2620 may be formed by etching, scribing or any other standard ornonstandard microfabrication techniques.

Preferably, counter-electrode 420 and notch 2620 are dimensioned suchthat counter-electrode 420 fits within the corresponding notch 2620 ofan adjacent solar cell 402 and touches or otherwise communicates withconductive core 404 without touching TCO layer 412 or semiconductorjunction 410 of the adjacent solar cell 402.

Further illustrated in FIG. 26A is a transparent electrically insulatingsubstrate 406 that covers all or a portion of face 2666 of the planararray. In some embodiments, solar cells 402 contact substrate 406. Insome embodiments, solar cells 402 do not contact substrate 406. In someembodiments, the plurality of elongated solar cells 402 are configuredto receive direct light from both face 2633 and face 2666 of the planararray. Solar cell assembly 2600 further comprises a transparentinsulating covering 422 disposed on face 2633 of the planar array,thereby encasing the plurality of elongated solar cells 402 between thetransparent insulating covering 422 and the transparent electricallyinsulating substrate 406. A sealant such as ethyl vinyl acetate (EVA) ispreferably used to seal substrates 406 onto solar cells 402 and to fillthe crevices between solar cells 402.

FIG. 26B provides a close up view of the region outlined by box 26B inFIG. 26A. Counter-electrode 420 may comprise one or more layers of anyconductive material. For example, in one embodiment, counter-electrode420 comprises a bead or strip 2622 of nickel, which may be coated with alayer 2624 of another conductive material such as aluminum, molybdenum,copper, steel, nickel, silver, gold, or an alloy thereof. In a preferredembodiment, layer 412 comprises zinc oxide and portion 2622 ofcounter-electrode 420 comprises nickel. In another preferred embodiment,layer 412 comprises indium tin oxide and portion 2622 ofcounter-electrode 420 comprises silver. In such embodiments, layer 2624may be any conductive material as described above.

In some embodiments counter-electrode 420 is a conductive tape, or acontact tape 2624 with a conductive bonding 2622. As shown in FIG. 26B,layer 2622, which may be a conductive bonding, preferably attaches toTCO layer 412, which can be made of material such as aluminum doped zincoxide, indium-zinc oxide, or indium-tin oxide, or other materials asdescribed herein. Exemplary conductive tapes include, but are notlimited to, ZTAPE available from iEC (Commerce City, Colo.). However, ingeneral, any conductive tape comprising an adhesive with a backing ontowhich is deposited a conductive material (e.g., silver, tin, nickel,copper, graphite, or aluminum) may be used as counter-electrode 420. Thebonding layer 2622 is preferably conductive and compatible with zincoxide and/or other materials used in TCO layer. In one embodiment,bonding layer 2622 is made of an optimized adhesive, for example asdescribed by Hertz et al., “CIGS Solar Modules Contacted by ConductingAdhesives and Ultrasonic Welding,” Proceedings of the 20^(th) EuropeanPhotovoltaic Solar Energy Conference, Barcelona, Spain, June 2005, p.1910-1913, which is hereby incorporated herein by reference in itsentirety.

As with other embodiments described herein, conductive core 404 may bemade of a uniform conductive material, or of a multi-layer core asdescribed in Section 5.4. For example, as shown in FIG. 26B, core 404may have a conductive or non-conductive inner core and a conductiveouter layer 2630 which is circumferentially disposed on the inner coreof 404. Outer layer 2630 may be made of any conductive material, suchas, for example, aluminum, molybdenum, steel, nickel, silver, gold, oran alloy thereof. Other suitable materials are described, for example,in Section 5.4.

FIG. 26C provides a cross-sectional view with respect to line 26C-26C ofFIG. 26A. Solar cells 402 are electrically connected to other in seriesby arranging the solar cells such that counter electrode 420 of eachsolar cell contacts conductive core 404 of adjacent solar cell, e.g.,through notch 2630. An electrical lead or wire 833 can be used as shownin FIG. 26C to provide an electrical connection between acounter-electrode 420 on one end of the solar cell assembly 2600 and theconductive core 404 on the other end of the solar cell assembly 2600.The separation distance between solar cells 402 is any distance thatprevents electrical contact between the TCO layers 412 of individualcells 402. For instance, in some embodiments, the distance betweenadjacent solar cells is 0.1 micron or greater, 0.5 microns or greater, 1micron or greater, 5 microns or greater, 100 microns or greater, or 0.1mm or greater.

The solar cell assembly 2600 illustrated in FIGS. 26A-C has severaladvantages. First, because of the positioning of counter-electrodes 420and the transparency of both substrate 406 and covering 422 (inpreferred embodiments), there is negligible shading of the assembly. Forinstance, the assembly can receive direct sunlight from both face 2633and face 2666. Second, in embodiments where a sealant such as ethylvinyl acetate (EVA) is used to laminate substrate 406 and covering 422onto the plurality of solar cells, the structure is completelyself-supporting. Still another advantage of the assembly is that is easyto manufacture. Unlike solar cells such as that depicted in FIG. 3A, nogrid of interconnects is needed to electrically connect the solar cellsin series.

Still another advantage of the assembly illustrated in FIG. 26 is thatthe distance that electric current must travel within counter-electrodes420 is substantially shorter than the distance electric current musttravel within counter-electrodes 420 of the other embodiments disclosedin previous sections. For example, in the assemblies described in thissection and illustrated in FIG. 26, electric current travels from onesolar cell 402 to an adjacent solar cell 402 through the cross-sectionalthickness of counter-electrode 420. In embodiments described in previoussections, electric current traverses the length of thecounter-electrodes 420. Because the current travels shorter distances inthe configurations described in this section, counter-electrodes 420 canbe made thinner than in configurations described in previous sections.Consequently, it is more economical to fabricate counter-electrodes 420our of choice materials (e.g., conductors such as metals or metalalloys) that are otherwise relatively expensive material. Such choicematerials include, but are not limited to, for example, a thin strip ofnickel or silver epoxy. Another advantage of the embodiments shown inFIGS. 26A-C is that the ends of solar cells 402 are substantially freeof counter-electrodes, leads, wires, and/or other connections. Thisarrangement simplifies manufacturing and packaging of assemblies 2600,and consequently may result in lower production costs.

Although not illustrated in FIG. 26, in some embodiments in accordancewith FIG. 26, there is an intrinsic layer circumferentially disposedbetween the semiconductor junction 410 and the transparent conductiveoxide 412 in an elongated solar cell 402 in the plurality of elongatedsolar cells 402. This intrinsic layer can be made of an undopedtransparent oxide such as zinc oxide, metal oxide, or any transparentmetal that is highly insulating. In some embodiments, the semiconductorjunction 410 of solar cells 402 in assembly 700 comprise an innercoaxial layer and an outer coaxial layer where the outer coaxial layercomprises a first conductivity type and the inner coaxial layercomprises a second, opposite, conductivity type. In an exemplaryembodiment the inner coaxial layer comprisescopper-indium-gallium-diselenide (CIGS) whereas the outer coaxial layercomprises CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments notillustrated by FIG. 26, the conductive cores 404 in solar cells 402 arehollowed.

In some embodiments, a solar cell assembly 2700 comprises one or moreinternal reflectors 1404 disposed between and in contact with thecounter electrode 420 and core 404 through notch or opening 2630, asshown in FIGS. 27A and B. Preferably internal reflector 1404 isconductive and in electrical communication with counter-electrode 420 ofone solar cell 402 and conductive core 404 of an adjacent solar cell402. For example, internal reflector 1404 may have an elongated astroidshape as shown in FIG. 27A, where one edge of each reflector 1404contacts counter electrode 420 of a solar cell 402, and the oppositeedge of reflector 1404 contacts core 404 through the notch or opening2620 in layers 410 and 412. In some such embodiments, internal reflector1404 is sealed to counter-electrode 420 of a first solar cell 402 andconductive core 404 of a second solar cell 402 by an electricallyconductive glue or ink. Reflector 1404 may have any of thecharacteristics, dimensions, materials, or uses as described herein, forexample in Sections 5.8 and 5.9.

In some embodiments, solar cells 402 contact substrate 406. In someembodiments, solar cells 402 do not contact substrate 406. As describedwith solar cell assembly 2600, a transparent electrically insulatingsubstrate 406 preferably covers all or a portion of face 2766 of theplanar array of solar cells. A sealant such as EVA, for example, sealsthe assembly to substrate 406. The sealant is preferably applied in aliquid form in order to reach into and fill all spaces and crevicesbetween solar cells 402 and reflectors 1404.

In some embodiments, solar cell assembly 2700 further comprises atransparent insulating covering 422 disposed on face 1420 of the planararray of solar cells 402, thereby encasing the plurality of elongatedsolar cells 402 and sealant 1702 between the transparent insulatingcovering 422 and the transparent electrically insulating substrate 406.In some embodiments, transparent insulating covering 422 and/or thetransparent insulating substrate 406 touch solar cells 420 and/orreflectors 1404. In other embodiments, transparent insulating covering422 and transparent insulating substrate 406 are separated from solarcells 420 and reflectors 1404 by sealant 1702.

There is no limitation on the cross-sectional shape of reflector 1404.In general, the cross-sectional shape of reflector 1404 is optimized toreflect as much light as possible onto the elongated solar cells 402.For example, in one embodiment, internal reflector 1404 has across-sectional shape that is four-sided, and at least one a side ofsaid four-sided cross-sectional shape is linear. In another embodiment,internal reflector 1404 has a cross-sectional shape that is four-sided,and a side of the four-sided cross-sectional shape is concave. Inanother embodiment, internal reflector 1404 has a cross-sectional shapethat is four-sided, and a side of the four-sided cross-sectional shapeis parabolic. In another embodiment, internal reflector 1404 has across-sectional shape that is four-sided, and a side of the four-sidedcross-sectional shape is circular or elliptical. In another embodiment,internal reflector 1404 has a cross-sectional shape that is four-sided,and a side of the four-sided cross-sectional shape defines a diffusesurface on the internal reflector.

7. References Cited

All references cited herein are incorporated herein by reference intheir entirety and for all purposes to the same extent as if eachindividual publication or patent or patent application was specificallyand individually indicated to be incorporated by reference in itsentirety for all purposes.

Many modifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. For example, in some embodiments the TCO 412 iscircumferentially coated with an antireflective coating. In someembodiments, this antireflective coating is made of MgF₂.Counter-electrodes 420 formed in divots in elongated solar cells 402have been described. However, the invention is not limited to such solarcell 402/counter-electrode 420 arrangements. Rather, in someembodiments, counter-electrodes 420 are formed on the sides of elongatedsolar cells 402 using evaporated metal. The specific embodimentsdescribed herein are offered by way of example only, and the inventionis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled.

1. A solar cell assembly comprising: (A) a plurality of elongated solarcells, each elongated solar cell in said plurality of elongated solarcells comprising: an elongated conductive core configured as a firstelectrode, a semiconductor junction disposed on said elongatedconductive core; a transparent conductive oxide layer disposed on saidsemiconductor junction; and an elongated counter-electrode attached toand extending from the transparent conductive oxide layer, wherein theelongated conductive core of each respective elongated solar cell in theplurality of elongated solar cells has an exposed portion not covered bythe semiconductor junction and the transparent conductive oxide layer ofthe respective elongated solar cell, and wherein the counter-electrodeof a first elongated solar cell in said plurality of elongated solarcells is in electrical communication with the exposed portion of theconductive core of a second elongated solar cell in said plurality ofelongated solar cells that is adjacent to the first elongated solarcell; and (B) an internal reflector that is configured between and incontact with the counter-electrode of the first elongated solar cell andthe exposed portion of the conductive core of the second elongated solarcell, such that a portion of the solar light reflected from the internalreflector is reflected onto the first and second elongated solar cells.2. The solar cell assembly of claim 1, wherein the internal reflectorhas a hollow core.
 3. The solar cell assembly of claim 1, wherein theinternal reflector comprises a plastic casing with a layer of reflectivematerial deposited on said plastic casing.
 4. The solar cell assembly ofclaim 3, wherein the layer of reflective material is polished aluminum,aluminum alloy, silver, nickel or steel.
 5. The solar cell assembly ofclaim 1, wherein the internal reflector is a single piece made out of areflective material.
 6. The solar cell assembly of claim 5, wherein thereflective material is polished aluminum, aluminum alloy, silver, nickelor steel.
 7. The solar cell assembly of claim 1, wherein the internalreflector comprises a plastic casing onto which is layered a metal foiltape.
 8. The solar cell assembly of claim 7, wherein the metal foil tapeis aluminum foil tape.
 9. The solar cell assembly of claim 1, wherein across-sectional shape of the internal reflector is astroid.
 10. Thesolar cell assembly of claim 1, wherein a cross-sectional shape of theinternal reflector is four-sided; and a side of said four-sidedcross-sectional shape is parabolic.
 11. The solar cell assembly of claim1, wherein a cross-sectional shape of the internal reflector isfour-sided; and a side of said four-sided cross-sectional shape isconcave.
 12. The solar cell assembly of claim 1, wherein across-sectional shape of the internal reflector is four-sided; and aside of said four-sided cross-sectional shape is circular or elliptical.13. The solar cell assembly of claim 1, wherein a cross-sectional shapeof the internal reflector is four-sided; and a side of said four-sidedcross-sectional shape defines a diffuse surface on said internalreflector.
 14. The solar cell assembly of claim 1, wherein the internalreflector has a first edge and a second edge, wherein said first edgeand said second edge run lengthwise along said internal reflector; andwherein (i) the first edge of the internal reflector contacts the firstelongated solar cell; and (ii) the second edge of the internal reflectorcontacts the second elongated solar cell.
 15. The solar cell assembly ofclaim 14, wherein said first edge is sealed to said first elongatedsolar cell and said second edge is sealed to said second elongated solarcell by an electrically conductive glue or ink.
 16. The solar cellassembly of claim 14, wherein said first edge is sealed to said firstelongated solar cell and said second edge is sealed to said secondelongated solar cell by a transparent insulator.
 17. The solar cellassembly of claim 16, wherein said transparent insulator is ethyl vinylacetate or spray Teflon.
 18. The solar cell assembly of claim 1, whereina cross-sectional shape of the internal reflector is four-sided andwherein a side of said four-sided cross-sectional shape is the involuteof the cross-sectional shape of the first elongated solar cell or thesecond elongated solar cell.